Media playback is unsupported on your device Media caption David Cameron: "The UN Security Council has reached its decision, there is a responsibility... to respond"

David Cameron has reacted to the Libyan government's offer of a ceasefire by saying he would judge Col Muammar Gaddafi "by his actions not his words".

The UK prime minister has said UK warplanes are being deployed to help protect the Libyan people from "brutal" attacks by the Gaddafi regime.

A joint UK, US and French operation is to be launched after the UN backed a resolution authorising a no-fly zone.

In response, the Libyan regime said it was calling an immediate ceasefire.

Libyan foreign minister Mussa Kussa said the country would abide by the UN resolution, which calls for an immediate ceasefire and an end to attacks on civilians, to stop "all military operations" and was prepared to allow UN observers into the country to verify its compliance.

People 'brutalised'

Reacting to the Libyan government's announcement, Mr Cameron said of Col Gaddafi: "We will judge him by his actions not his words.

"What is absolutely clear is the UN Security Council resolution said he must stop what he is doing, brutalising his people. If not, all necessary measures can follow to make him stop," he told BBC News.

"That is what we agreed last night, that is what we are preparing for and we'll judge him by what he does."

Later, a joint statement by the UK, France and the US demanded that Gaddafi's troops stop their advance on Benghazi and pull back from the towns on Ajdabiya, Misrata, and Zawiya.

It also said water, electricity and gas supplies must be re-established to all areas, and humanitarian aid allowed to reach the Libyan people.

These terms were non-negotiable, the statement added.

A French foreign ministry spokesman said his government was "very cautious" about the ceasefire commitment as "the threat on the ground" in Libya had not changed.

US Secretary of State Hillary Clinton said the US wanted to see pro-Gaddafi forces withdraw a "significant distance away" from their current positions in the east of the country close to the opposition stronghold of Benghazi.

Mr Cameron will discuss the international community's next steps with French President Nicolas Sarkozy and Arab leaders in Paris on Saturday.

The BBC's Europe editor Gavin Hewitt said the ceasefire offer had "changed the equation" and he did not get the impression that military action was imminent - as seemed the case before. Mr Cameron and other leaders had to decide how to "test" whether the ceasefire was being upheld, he added.

'Legal basis'

Mr Cameron earlier told MPs that there was a "clear and unequivocal" legal basis for action in Libya if the Gaddafi regime did not comply and the UK would "play its part" in enforcing the will of the international community.

"The defence secretary and I have now instructed the chief of the defence staff to work urgently with our allies to put in place the appropriate military measures to enforce the resolution - including a no-fly zone," he said.

"Preparations to deploy these aircraft have already started and in the coming hours they will move to airbases from where they can start to take the necessary action."

Media playback is unsupported on your device Media caption Moussa Koussa announced a ceasefire earlier in March, after the UN resolution was passed

He insisted the UN resolution was "very clear" in its aims to bring hostilities in Libya to an end and to protect civilians from a possible "bloodbath" in Benghazi.

Asked whether the UN agreement endorsed regime change in Libya, he said he continued to believe the country had no future with Col Gaddafi in power and this view was widely shared by the UK's allies.

"Any decision to put the men and women of our armed forces into harm's way should only be taken when absolutely necessary," he said. "But I believe that we cannot stand back and let a dictator whose people have rejected him kill his people indiscriminately. To do so would send a chilling signal to others."

Military preparations

The Tornado GR4 fighter jet, equipped with precision weapons, is among the first military assets the UK could use to defend a no-fly zone.

The planes are stationed at RAF Marham and RAF Lossiemouth, though it is not yet clear which military base they would ultimately fly from. Options include bases in southern France, southern Italy or RAF Akrotiri in Cyprus, or - with permission - Malta.

Resolution 1973 Image caption The UK (middle) was one of the 10 members of the UN Security Council to vote for the resolution Imposes "ban on all flights in Libyan airspace" except for aid planes

Authorises member states to "take all necessary measures" to "protect civilians and civilian populated areas under threat of attack"

Excludes occupation force

Toughens arms embargo by calling on all member states to "inspect, in their territory, vessels and aircraft bound to or from Libya"

Widens asset freeze to include Libyan Investment Authority, Central Bank of Libya and Libyan National Oil Company among others Analysis: PM's Libya policy risky In full: UN resolution on Libya In quotes: UK reaction In quotes: International reaction

Typhoons warplanes, based at RAF Coningsby and RAF Leuchars, are most likely to be used in an air-to-air combat role. UK reconnaissance and refuelling planes are also being deployed.

Before addressing Parliament, the prime minister chaired an emergency meeting of the cabinet, attended by military commanders.

MPs are to get the chance to vote on the UK's potential involvement at the end of a Commons debate on Monday. Mr Cameron said there would be "full parliamentary scrutiny" of the issue and MPs would have access to a summary of Attorney General Dominic Grieve's legal advice.

The Conservatives' Lib Dem coalition partners are supporting the action, saying it is a wholly different situation from the 2003 Iraq war which they opposed. Labour leader Ed Miliband also gave his party's backing, saying the world could not "stand by and do nothing" in the face of the violence in Libya.

The UN resolution imposes a ban on all flights in Libyan airspace, excluding aid flights, and authorises member states to "take all necessary measures" to "protect civilians and civilian populated areas under threat of attack" short of putting an "occupation force" on the ground.

It would permit air strikes on Libyan ground troops or allow attacks on Libyan war ships if they were attacking civilians.