The Tumen River, which divides China from North Korea, was frozen solid. We peered across to the North Korean side: the border guards' wooden huts emitted no smoke, suggesting that they were unheated. Outside, it was -16° Celsius. A wolf-like dog scampered along the river, leaving pad-prints in the snow. There were human footprints, too - evidence of a patrol, maybe, or of refugees who had fled at night, so desperate to leave North Korea that they would risk freezing to death or being shot.
Some chance it across the river on their own, hiding in the mountains on the Chinese side, hoping to find farm work in exchange for food. A small number manage to link up with church networks that smuggle people through China and eventually to South Korea. For many women, however, the choice is stark: to die of hunger in North Korea or be sold into slavery in China.
So poor are farmers in this remote part of Manchuria that local women refuse to marry them, preferring to try their luck in the small town of Huanqing or the nearby city of Yanji. As an alternative, Chinese families buy North Korean women from the snakehead gangs that operate along the Tumen and Yalu Rivers. With the one-child policy now creating an overall shortage of young women in China, the demand for North Korean refugee brides is certain to increase.
The driver took us 15 miles inside China to a bleak, snow-cloaked village. The only living creatures outside, under the leaden grey sky, were a few tethered cattle. It was the middle of the morning, but the village was silent. A wooden window frame swung unevenly on rusting hinges; snow covered the piles of firewood stacked against barn walls.
We walked up a frozen mud pathway to a shabby brick house, thatched with twigs. Inside, it was warm, the concrete floor covered in yellow plastic sheeting and heated by a wood burner underneath. Two women sat on the hot floor among the men who had bought them. The younger one was pretty, her smooth hair pulled back in a ponytail, and she smiled as she played with her five-year-old son. The driver had told us earlier that she had cost the equivalent of £130. The older one, who said she was 52, must have been cheaper, because she was already beyond childbearing age when she arrived. Her function was to cook and work in the fields. She had a mouthful of gold teeth. Neither would reveal her name or allow her face to be photographed.
"If the North Koreans find out I'm here, they will say I'm a traitor and they'll kill me," said the younger one. The older woman told her story first. "When I was in North Korea, I was sick and couldn't work, " she said. "My husband had died. The government put me and my children in a camp. When my son died, I was with him. There wasn't enough food, so he went to scavenge whatever people threw away on the streets and ate it. He got sick from eating garbage. He was in the hospital for five days before he died. He was 17. Now I have no family left in North Korea." Her reasoning was simple. "I escaped from North Korea because there was no food. My husband died. My son died. I didn't want to die, so I came to China."
It is hard to imagine a situation so desperate that you would conclude being sold was your least worst option, but that was her decision. "Someone in North Korea told me that I could come to China and marry into a Chinese family," she said. "I crossed the Tumen River and lived with a family in Tumen town for three days. I said that I wanted to be far away from the border, and they told me people are bought and sold in Shandong, so they sent me there."
She said a North Korean man had sold her to a Han Chinese family, but she could not speak the language and grew ever more miserable. "I lived there for three months but I couldn't talk to anybody in that Han family. They didn't give me a lot of food either. I worked on the farm. Then somebody brought me here to this family. I don't know how much money he sold me for. Those people are middlemen."
At this point our driver interrupted. "Don't use the word 'sold'," he said. "Just say you got married." He had told us he knew where the North Koreans were hiding, because he and his brother sometimes picked them up at the border and drove them inland, or were hired to take them further afield. We wondered if he was employed by the traffickers; driving journalists to meet the refugees was just another part of the business.
The younger woman was reluctant to talk. "I've lived in this village for five years, and I'm afraid to be caught by the police," she said. She indicated our camera, which we were carefully not pointing at her face. "I'm afraid of television, too," she said, looking at the floor.
The police had caught her when she was six months pregnant, but her new family paid a bribe to stop her being deported. "I think the police let me go because I was pregnant."
Refugees who have reached South Korea tell stories of pregnant women sent back to North Korea from China being forced to abort or even murder their newborns. Although many Chinese families in the border area are ethnically Korean, the regime in Pyongyang is obsessed with racial purity and believes that the children of Chinese fathers are contaminated. "Whenever I hear police cars, my heart jumps, and when they come, I just run out and hide," said the younger woman. "As long as I'm not caught by the police, I'll just stay here with my son and live a quiet life. Life is not easy in China, but I just want to live like this, and not get caught."
The Chinese government regards the North Koreans not as refugees needing help, but as illegal aliens. It will not allow the UN High Commissioner for Refugees (UNHCR) access to the border area, and limits the number of South Korean diplomats in the region, for fear they will encourage North Koreans to seek asylum. "The Chinese government flat-out refuses to recognise North Koreans as refugees," says Ron Redmond, chief spokesman for UNHCR. "But of course we help them; we've helped North Koreans reach South Korea. Nobody knows how many refugees there are: estimates have ranged from 30,000 to 300,000. " In 2005 an American Christian pastor, Philip Buck, was arrested in Yanji and held for 18 months on charges of "people smuggling". He was part of the "Seoul train" that takes refugees out of North Korea and China through Mongolia and Thailand to safety.
The Chinese authorities detain North Koreans in a monolithic yellow concrete block with small windows staring out across the river from a hilltop just outside Tumen. Every few weeks they load detainees into vans and take them across the bridge, where they are handed over to the border guards under a portrait of North Korea's "President for Eternity", Kim Il-sung, who died in 1994.
In the late 1990s, hunger in North Korea became so acute that the country appealed for food aid and the UN World Food Programme stepped in. It is estimated that roughly two million people died. Now China and South Korea donate food, most of which is believed to reach the military and the elite in Pyongyang, while the WFP continues a smaller programme. "We have identified 1.9 million people dependent on food aid in North Korea," says Robin Lodge, a WFP spokesman. "At present we are feeding 740,000. Our funding is such that we believe we can continue feeding them until June this year."
Sanctions worsen hunger
In an anonymous block of flats in the mean little town of Huanqing, we met a family - mother, father, teenage son and daughter - that had been caught trying to take food to relatives back home. A Chinese Christian who would give his name only as "Mr Kim" took us to the flat which served as their refuge and their prison. "We're fine here, except we dare not go out," said the mother. She said Mr Kim brought them food and other necessities.
For five years the family had lived as a group of itinerant beggars, wandering around North Korea looking for food, because food distribution at the coal mine where the father had worked was inadequate. Their memory of dates and details was vague, but they said they crossed over into China in 2005. "For almost a year we lived in a tent in the mountains," said the father. They found the Christian network and converted. Then they began to worry about relatives back home, so they decided to join other Christians taking food back across the border.
Within days they were caught. Under interrogation, their friends confessed to being Christians and were jailed as political prisoners. The women were released and the father managed to resist confession, but the son was transferred back to their home town of Gosang.
"I was detained for another 20 days. There were 150 people in a cell 150 square metres large," he said. "Men were kept in the cell and women in the corridor." They were fed, but that was not enough. "Many prisoners got skin diseases," he said. "There was no medicine, and I saw many dead people in the cell."
It was not clear why he had been released, nor how the family had been reunited in China. Now Mr Kim was hoping to smuggle all four of them to Beijing and eventually to South Korea. When asked about relatives back home, mother and daughter started to cry. Although UN sanctions imposed after North Korea's nuclear test last year target luxury goods, Mr Kim's concern was that further economic pressure could send starvation levels back to those of the late 1990s. "The other day, I met some North Koreans who have only just arrived," he said. "They are ready for another economic crisis. There will be a second Hardship Long March this year."
In the past few years the North Korean economy has been prised open and increasing numbers of Chinese traders are doing business in Pyongyang, but the Chinese fear that if Japan and the US move too harshly against North Korea over its nuclear programme, the regime will become unstable and millions will surge across the border. "If more refugees flee to China, the Chinese government will send them back," said Mr Kim. "They won't change their policies."
On the outskirts of the village, we tramped through the snow to meet a family of three that had arrived last spring and had been living in the mountains, working in exchange for food on a farm where they catch and kill toads for traditional medicine. The parents told a now familiar story: their son had died of starvation last year, so they decided to come to China with their remaining child, a 13-year-old girl. As we talked, she played with a puppy - her only entertainment, because she cannot go to school in China.
No one knows how many other children like her are hiding in the villages and the mountains of north-eastern China, condemned to live as permanent fugitives for the crime of trying to survive.
Lindsey Hilsum is China correspondent for "Channel 4 News"
North Korea: the facts of life
Healthcare and education are provided according to government assessment of an individual's and family's political loyalty.
Usually only children of the elite are allowed to go to college and hold prominent jobs.
Between 1996 and 2005 more than $2bn of food aid was delivered to NK.
37 per cent of young children are clinically malnourished.
Approximately one-third of mothers are malnourished and anaemic.
A citizen can be sentenced, without judicial process, to a life of "hard labour" in mining, timber-cutting, or farming.
In 2003, the government announced that it would refrain from executing criminals in public.
In 2004, a government campaign called on men to keep their hair short, stressing the "negative effects" of long hair on "human intelligence development".
Research by Lucy Knight
Sources: Human Rights Watch, Amnesty, WFP