The United Nations Security Council has voted in favour of a resolution to impose a no-fly zone on Libya. Here is a guide to exactly what this means.
What has been agreed?
The resolution (full text here) authorises member states to "take all necessary measures" to "protect civilians and civilian-populated areas under threat of attack", in particular Benghazi, the rebel stronghold in the east of the country, which is mentioned by name.
It also calls for an immediate ceasefire, an end to the violence, measures to make it more difficult for foreign mercenaries to get into Libya and a tightening of sanctions.
What does this mean in practice?
The resolution would permit air strikes on Libyan ground troops or allow attacks on Libyan warships if they were attacking civilians.
Hillary Clinton, the US secretary of state, said a no-fly zone would "require certain actions taken to protect the planes and the pilots, including bombing targets like the Libyan defence systems".
While three air strikes were reported on the outskirts of Benghazi on Thursday, including at the airport, as well as another air raid further south, the dictator's troops have largely been relying on tanks and other ground forces during assaults on the rebels. UN planes could also attempt to bomb tanks and artillery, but unless they have very specific information, this risks causing rather than preventing civilian casualties.
When will action be taken?
According to the French prime minister, François Fillon, military action could begin "within hours". Throughout the past few weeks, France has been an advocate of more aggressive action in Libya.
Despite Fillon's assertion that "time is of the essence", a US military official has said that no immediate US action is expected.
No 10 sources have refused to put a timetable on British military engagement. Reportedly, the British Ministry of Defence is still finalising contingency plans. RAF ground attack aircraft are ready to be mobilised, and should be in action within days. According to analysts at the International Institute for Strategic Studies (IISS), Britain and France could operate a limited no-fly zone over Benghazi with little or no US support.
Ahmed el-Gallal, a Libyan opposition co-ordinator, said he hoped that the resolution would be enforced "immediately".
Which countries are involved?
The UN resolution was co-sponsored by Britain, France and Lebanon, with the US heavily involved in the drafting, and was passed by 10 votes to 0, with five countries abstaining, including Russia, China and Germany.
Calls for a no-fly zone have been led by Britain and France, and these two countries are likely to take the lead at least initially. As mentioned above, they could have sufficient military capability to mount a limited no-fly zone should the US choose not to get involved, though this may not have the desired impact on Muammar al-Gaddafi's crackdown.
Many western countries have significant military assets nearby, including aircraft carriers in the Mediterranean, a large US airbase in Italy and a large British airbase in Cyprus.
Arab League and Gulf states have warships and fighter plans, but the extent of their involvement is as yet unclear. Reportedly two Arab nations have promised to take part in intervention, though it is not clear which countries. This could take the form of opening airspace, intercepting Libyan shipping, or contributing their own strike aircraft.
Canada has said it will send warplanes to help enforce the resolution. Nato would have to meet before committing any forces.
Why has it been passed now?
Gaddafi's forces have recently retaken several towns seized by rebels during the uprising, and are advancing on the stronghold of Benghazi. Without foreign intervention, the rebels could be crushed by the weekend.
While David Cameron and Nicolas Sarkozy have been calling for a no-fly zone for days, the US getting behind the plan was key to it passing. After weeks of stalling, Washington backed the resolution, following calls for a no-fly zone by the Arab League at the weekend. This call for foreign intervention is unprecendented. It is a significant change in strategy for the US, which depends on any action being multilateral rather than US-led.
Another diplomatic shift that was key to the vote passing was the abstention of countries that do not favour military action. It was feared earlier that China and Russia would exercise their right of veto.
Will it stop Gaddafi?
The colonel's son Saif has been quoted this morning as saying that Libya is "not afraid" of UN-backed action. Saif called the vote a sign of "flagrant colonisation" and warned of dire consequences. "This is craziness, madness, arrogance," he told the Portuguese TV channel RTP. "If the world gets crazy with us, we will get crazy, too. We will respond."
Speaking on the Today programme, Libya's rebel deputy envoy to the UN, Ibrahim Dabashi, expressed doubt that the international intervention would change the situation on the ground.
As we have already seen, Gaddafi will not go down without a fight.
Is it likely to escalate into full-blown war?
It is entirely possible that Gaddafi will retaliate. In a statement broadcast on Libyan television, the defence ministry said: "Any foreign military act against Libya will expose all air and maritime traffic in the Mediterranean Sea to danger and civilian and military [facilities] will become targets of Libya's counterattack." As mentioned above, several western countries have military assets nearby.
However, the UN resolution specifically rules out sending a foreign occupation force into any part of Libya. A ground intervention would require a second resolution to be passed.