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David Cameron has urged the UN to "show leadership" ahead of negotiations about whether to back a no-fly zone in Libya.

The UK, France and Lebanon are pressing for the move to stop air attacks on civilians as fighting intensifies.

The UK prime minister said a no-fly zone was not a "simple solution" to the Libyan crisis but one of a series of steps that was needed to "make sure we get rid of this regime".

China and Russia are among countries sceptical about a no-fly zone.

Opposition leaders have appealed for international help in limiting Col Gaddafi's resources as his forces maintain their onslaught on rebel positions in the east of the country and as Saif Gaddafi, son of the Libyan leader, claimed the uprising "will be over in 48 hours".

Anti-Gaddafi forces have denied claims their opponents have taken Ajdabiya - the last town before their headquarters in Benghazi - following a ground assault on the town.

The UK is increasing the pressure for a no-fly zone over Libya by tabling the draft resolution at the UN Security Council. France, which along with Lebanon is also sponsoring the resolution, has said it is confident it will "achieve its objective".

Germany, Russia and China are said to be among those opposing such proposals, aimed at preventing air attacks on rebels by pro-Gaddafi forces, while there is increasing pressure on the US to state its position.

The Arab League has backed the idea but a meeting of G8 foreign ministers in Paris failed to do so.

Mr Cameron authorised the formal proposal of the text - which contains a "menu" of options for restraining Col Gaddafi's regime.

Asked about the issue at Prime Minister's Questions, Mr Cameron said: "At the end of this if Col Gaddafi is left in place that would send a terrible message - not just to people in Libya - but others across the region, who want to see greater democracy and openness in their societies. That is why it is right for Britain to play that leading role at the UN and elsewhere."

But amid uncertainty over US support for the resolution and whether progress would be made, ´óÏó´«Ã½ political editor Nick Robinson said the "political blame game" about the world's response to the crisis had already begun.

Foreign Secretary William Hague suggested on Wednesday that if a new resolution was blocked, then the UK and others could use an existing doctrine to protect civilian life to justify military action.

"It is on the table," he told MPs on the Foreign Affairs Select Committee.

Mr Hague accepted that the doctrine supporting intervention to protect civilian life - known as the "duty to protect" - was a legal "grey area" and a relatively new concept without much case law.

But he added: "If I am asked, are there some circumstances in which countries have the right to act, and it is legal even without a Security Council resolution? From the legal advice I have received in office, there are some circumstances."

But he emphasised that it could take place only if there was demonstrable need, a clear legal basis and regional support, including the "active participation" of Arab countries.

Mr Hague - who said the uprisings across North Africa and the Middle East were "rapidly becoming" the most important event of the 21st Century - also suggested talk of a no-fly zone may have influenced Col Gaddafi's military tactics in recent days and deterred "even worse behaviour than we have seen".

Nato defence ministers have said a clear UN mandate would be needed for them to get involved in Libya, but Russia has said the Arab League had not "formalised" its decision to back a no-fly zone.

Germany has advised against military intervention, warning of a "slippery slope" to a full-scale war.

The draft resolution - which France hopes to put to a vote by Friday - would establish a ban on all flights in Libya, authorise member states to enforce it and call on them to participate in it.

It also urges stronger enforcement of the arms embargo, adds names, companies and entities to the sanctions list, bans commercial flights from bringing arms and mercenaries into Libya and would set up an expert panel to monitor implementation.

Meanwhile, Saif Gaddafi has said government forces were "close to" the rebel stronghold of Benghazi and a no-fly zone would make no difference.

"The military operations are finished," he told French TV channel Euronews. "In 48 hours everything will be over. Our forces are close to Benghazi. Whatever decision is taken, it will be too late."

But the ´óÏó´«Ã½'s John Leyne, who is in the city, said he did not believe the government had the manpower to capture the city at this stage.