A source at UN headquarters in New York said military forces could be deployed "within hours" of a new security council resolution calling for states to protect civilians by halting attacks by Muammar Gaddafi's forces by air, land and sea.
The resolution would impose a no-fly zone over Libya – but a no-fly zone was no longer enough, the source said. "The resolution authorises air strikes against tank columns advancing on Benghazi or engaging naval ships bombarding Benghazi," he said.
Britain, France and Lebanon sponsored the new resolution, which provides the moral and legal basis for military action.
British and French forces are understood to have been placed on standby after the US said it was prepared to support the measure if Arab countries agreed to take an active role.
The security council was scheduled to vote on the new resolution this evening, and its backers expressed confidence it would pass after hours of negotiation.
In London, William Hague, the foreign secretary, indicated to MPs that military preparations to protect Benghazi were at an advanced stage. The no-fly zone would be imposed from land, and not from aircraft carriers.
"No, it is not the case that carrier-borne aircraft are necessary to do such a thing," Hague said. "In the contingency plans of all the nations, none of them involve an aircraft carrier."
The increase in military preparations came as Gaddafi announced that his forces would invade Benghazi tonight and would show no mercy on fighters who resisted them.
"No more fear, no more hesitation, the moment of truth has come," he declared. "There will be no mercy. Our troops will be coming to Benghazi tonight."
Residents and a rebel spokesman reported three air strikes on the outskirts of the city, including at the airport, and another air raid further south.
There was also heavy fighting in residential areas of nearby Ajdabiyah, where around 30 people were killed, Al Arabiya reported.
Libyan authorities also warned that all maritime traffic in the Mediterranean would be in danger if it was targeted by foreign forces.
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," the statement said. "The Mediterranean basin will face danger not just in the short-term, but also in the long-term."
The UN resolution, which calls for "all necessary measures short of an occupation force" to protect civilians, needs the support of a further six further members of the security council to pass – and to avoid vetoes from Russia and China.
A Downing Street spokesman said: "The prime minister has been making a series of calls on Libya. He has spoken to a number of Arab and African leaders. We can now confirm that he has also spoken to several European leaders.
"In all his calls, the prime minister has made the case for strong action by the UN security council, to increase the pressure on Gaddafi and put a stop to the campaign he is waging against the Libyan people. The prime minister will be making further calls this evening."
The move marks a last-gasp attempt to keep the Libyan uprising alive.
It has been relatively rare in recent years for the UN to give the go-ahead for military action – the security council, for example, refused to support the Iraq invasion. The resolution reflects the extent of despair felt in Britain, France, the US and parts of the Arab world at the prospect of total victory by Gaddafi and fears of a massacre in Benghazi.
After weeks of prevarication by the US, Washington backed the resolution. The Obama administration was stalled by a split between the US secretary of state, Hillary Clinton, who favoured a no-fly zone, and the defence secretary, Robert Gates, who was opposed. The White House, caught in the middle, dithered.
Gates redeployed US naval vessels close to the Libyan coast and told Barack Obama that, though heavily engaged in Iraq and Afghanistan, the military was capable of fighting on a third front.
The US, crucially, insisted it would only act if there was Arab support, in order to avoid it being seen as a western intervention. Several Arab countries have promised to provide planes, but insisted upon their identity being withheld until the resolution was passed.
Speculation as to which countries would participate include Saudi Arabia, the United Arab Emirates and Qatar.
There is no plan to send in ground troops, other than for isolated incidents such as rescuing downed pilots.
Supporters of the resolution, speaking before the vote, said they were confident of achieving the necessary nine votes in the 15-member chamber. A source who was present at the talks said that China and Russia have vetoes that could scupper the resolution, but indicated they would abstain.
Brazil, Germany and India expressed scepticism over military action, but their votes were not needed to secure a majority.
John Kerry, the chairman of the US Senate foreign affairs committee, said: "The international community cannot simply watch from the sidelines as the Libyan people's quest for democratic reform is met with violence … Time is running out for the Libyan people. The world needs to respond immediately."