The Turkish prime minister, Recep Tayyip Erdogan, has stepped up his rhetoric against Syria, saying that the two countries are "not far" from war.
On Wednesday, five people were killed in a Turkish village by shelling across the border.
Today, Erdogan told a crowd in Istanbul:
"We are not interested in war, but we're not far from it either. This nation has come to where it is today having gone through intercontinental wars."
As the two countries squared up, the Syrian city of Homs came under a severe shell attack, its worst in several months. According to activists, there have also been attacks on Aleppo, Damascus, Hama and Idlin.
Abu Rami, an activist who was in Homs at the time of the shelling, told Associated Press: "Around dawn, the regime went crazy and started shelling hysterically. An average of five rockets a minute are falling."
Homs had been the focus of intense attacks from government forces up until April, when focus had shifted away from the city. However, on Friday there were attacks on several districts in Homs, including the neighbourhood of Khaldiya, Old Homs, Qusour and Jouret el-Shayah.
Rami Abdel Rahmen, of human rights group the Syrian Observatory for Human Rights, told AFP news: "It seems like the regime has a limited window to use its warplanes, because it is throwing everything it can at the rebels in Homs."