British Prime Minister David Cameron and French President Nicolas Sarkozy today wrote a joint letter to European allies, urging them to be ready for "all possible contingencies" in Libya, including the possible imposition of a no-fly zone to prevent air attacks by dictator Muammar Gaddafi.

The letter, on the eve of an emergency summit of European Union leaders in Brussels, condemned the Libyan authorities use of airpower in attacks on their own people as "unacceptable".

And Mr Cameron and Mr Sarkozy called on the EU to unite in calling for Gaddafi to go, arguing that his regime has "lost any legitimacy it may have once had".

"Since the Libyan people have started to rise against Muammar Gaddafi's brutal regime, the world is witnessing on a daily basis an unacceptable continuation of violence and repression in Libya," wrote the two men in a letter to European Council President Herman van Rumpoy, copied to the other 25 EU leaders.

"Ignoring UN Security Council resolution 1970 demands as well as calls from regional organisations and the whole international community, Gaddafi's regime continues to attack his own people, including with aircraft and helicopters. It is clear to us that the regime has lost any legitimacy it may have once had.

"This deliberate use of military force against civilians is utterly unacceptable. As warned by the Security Council, these acts may amount to crimes against humanity. All those involved in deciding, planning or executing such actions must know that they will be held accountable."

They called on EU partners, Nato allies and friends in the Arab world and Africa to unite in calling on Gaddafi to go and sending a clear signal that the Interim Transitional National Council in opposition stronghold Benghazi are "valid political interlocutors" in preparing for the aftermath of his departure.

They called for an immediate end to the use of force by Gaddafi against civilians and said: "We support continued planning to be ready to provide support for all possible contingencies as the situation evolves on the basis of demonstrable need, a clear legal basis and firm regional support.

"This could include a no-fly zone or other options against air attacks, working with Allies and partners, especially those in the region."

Tomorrow's summit comes amid signs that forces loyal to Gaddafi are preparing to step up their use of violence to crush the uprising, which has torn much of the country from his grasp.

In a televised interview, Gaddafi's son Saif said thousands of Libyans had volunteered to fight against rebels, whom he described as terrorists and armed gangsters.

"It's time for liberation. It's time for action. We are moving now," he said. "Time is out now. It's time for action. We gave them two weeks.

"Now it's too late for them. We are so united, we are so strong. And Libya will be free and peaceful soon."

Foreign Secretary William Hague accused Saif of "declaring war against his own people".

And he warned that the international community could move "very, very quickly" to send warplanes to protect the Libyan population from continuing aerial bombardment

Following meetings in Brussels of EU foreign ministers and Nato defence ministers, Mr Hague said preparations for imposing a military no-fly zone over Libya were in a "very advanced stage".

He said if evidence emerged of a major atrocity committed by the Libyan dictator against his own people, the international demand for action could swiftly pick up.

"Clearly if there were to be large-scale bombing attacks which the world could see and understand and could be verified on civilians, on populated areas in Libya, then that would massively strengthen the case for the introduction of a no-fly zone," he told the BBC.

"It can now be done very, very quickly because Britain and France have done the work at the UN Security Council on preparing a resolution, which we would then put for agreement with other nations.

"Nato is doing the planning on, in practice, what would you do, although of course it may involve many other nations if it happens apart from Nato. I think we are at a very advanced stage of that."

Mr Cameron and Mr Sarkozy urged all countries to implement the UN arms embargo on Libya and said this should include measures to prevent the departure of planes or convoys carrying mercenaries to fight for Gaddafi.

They confirmed the commitment of Britain and France to "the sovereignty, independence, territorial integrity and national unity of Libya" and their support for the desire of the country's people to choose their own leaders.

"When the Libyan people win their fundamental rights, we should be ready to support them with the necessary assistance and cooperation, in order to promote stability and development in Libya, for the benefit of all Libyans," they wrote.

The two leaders called on the UN to monitor the humanitarian situation closely and restated their backing for the investigation into possible abuses launched by the International Criminal Court.

Pressure on the Gaddafi regime was ratcheted up in a flurry of diplomatic activity in Brussels today, which saw the EU extend its sanctions to freeze the assets of five financial institutions.

Nato announced it was repositioning warships in the Mediterranean to improve monitoring of the situation.

But Secretary General Anders Fogh Rasmussen struck a cautious tone on further military action, saying defence ministers had simply discussed the "initial options" for a no-fly zone.

He also again emphasised there would have to be a clear mandate from the UN Security Council - a point echoed by US Defence Secretary Roberts Gates who said the existing resolution 1970 did not authorise the use of force.

"If there were to be a need for enforcement, there would need to be another UN Security Council resolution," he said.

Currently Russia, as one of the five permanent members of the Security Council with the power of veto over any new resolution, has expressed its strong opposition to any foreign military intervention in Libya.

US Secretary of State Hillary Clinton confirmed she will meet members of the Libyan opposition, both in the US and during visits to Egypt and Tunisia next week. But she cautioned that any use of military force by the US would require international authorisation, telling a congressional committee that unilateral action could have "unforeseeable" consequences.

Reports from Libya suggested fighting was intensifying, as forces loyal to Gaddafi assaulted towns deep in opposition territory.

Government tanks fired on rebel positions in the port of Ras Lanuf, while insurgents reported that warplanes had bombarded the oil town of Brega.

Officials in capital Tripoli said a Guardian reporter who went missing four days ago is being held by the Libyan authorities.

Iraqi national Ghaith Abdul-Ahad and Brazilian journalist Andrei Netto are understood to have been detained close to the coastal town of Sabratha on Monday.

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