Britain has been pressing with key allies for wide-ranging powers to intervene in Libya amid warnings that Colonel Muammar Gaddafi's forces are closing in on the key rebel stronghold of Benghazi.

The diplomats at the United Nations (UN) Security Council in New York were debating a strongly-worded draft resolution calling for "all necessary measures short of an occupation force" to protect civilians under threat of attack from the regime.

The revised text was tabled with United States support following a dramatic overnight change of heart by the administration in Washington.

After days of stalling over British and French calls for a military no-fly zone, the US ambassador to the UN, Susan Rice, signalled that they were now prepared to go further and back air strikes to halt the regime's advance.

"There is now nothing holding them back," Foreign Secretary William Hague told MPs during a Commons debate on north Africa and the Middle East.

It remained unclear whether Russia and China - who have been opposed to any foreign military intervention in Libya - would use their vetoes to block a resolution.

The US has said that it is only prepared to intervene with the authorisation of the Security Council and the participation of Arab League nations who backed the calls for a no-fly zone.

"I must not pretend that agreement on this will be easy, even on large elements of it," Mr Hague said. "We will do our utmost to ensure the passing of a resolution which places the maximum pressure on the Libyan regime and which extends protection to the beleaguered and oppressed civilian population of Libya."

Downing Street said that David Cameron was "working the phones", with calls to a series of Arab and African leaders to make what was described as a "strong case" for action at the Security Council.

"Clearly there is a race against time at the present time and the situation on the ground is increasingly concerning," the Prime Minister's spokesman said.