British, French and US military aircraft are preparing to defend the Libyan rebel stronghold of Benghazi after Washington said it was ready to support a no-fly zone and air strikes against Muammar Gaddafi's forces.

Jets could take off from French military bases if a no-fly zone is approved in a fresh United Nations security council resolution authorising "all necessary measures short of an occupation force" to protect civilians.

France, which has led the calls for a no-fly zone along with Britain, has offered the use of military bases on its Mediterranean coast about 750 miles from the Libyan coast. Several Arab countries would join the operation.

The finalising of military preparations came as Gaddafi's forces closed in on Benghazi and warned that they would target all maritime traffic in the Mediterranean if they are 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 Mediterranean basin will face danger not just in the short-term, but also in the long-term."

Gaddafi has warned Libyan rebels that his forces will invade Benghazi and show no mercy to fighters who resisted them. "No more fear, no more hesitation, the moment of truth has come," the Libyan leader declared, as he warned Benghazi residents that soldiers would search every house in the city and people who had no arms had no reason to fear.

"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 on Thursday, including at the airport, and another air raid further south. There was also heavy fighting in residential areas of nearby Ajdabiya, where around 30 people were killed, the TV station al-Arabiya reported.

The increase in Libyan rhetoric came as diplomats intensified their negotiations over a fresh UN security council resolution, tabled by Britain, France and Lebanon, to authorise a no-fly zone. The three countries need the support of a further six further members of the security council – and to avoid vetoes from Russia and China – to pass the resolution.

A security council source said the resolution would impose a no-fly zone over Libya but that was no longer enough. "The resolution authorises air strikes against tank columns advancing on Benghazi or engaging naval ships bombarding Benghazi," he said.

A source at UN headquarters in New York said military forces could be in action soon after a security council resolution calling for states to protect civilians by halting attacks by Gaddafi's forces by air, land and sea. Nato would have to meet after the vote to review the military planning that has already been completed.

The security council was scheduled to vote on the new resolution on Thursday evening and its backers expressed confidence that it would go through after hours of negotiation earlier in the day. The move marks a last-gasp attempt to keep the Libyan uprising alive.

David Cameron spoke to leaders of Arab countries on Wednesday night and on Thursday to persuade them to take part. The US had demanded Arab involvement to ensure that the west could not be accused of imposing its will on the Arab world. The prime minister also spoke to African and European leaders. Nigeria, Gabon, South Africa and Germany currently have seats on the security council.

Speaking outside the UN security council in New York, Alain Juppé, the French foreign minister, said there was "reason to anticipate that some Arab countries will participate." But he said that a land invasion was out of the question. "For us and in the resolution itself there is no question of having people on the ground in Libya."

Germany, which is opposed to a no-fly zone, is expected to abstain. In an interview with the Guardian, Germany's foreign minister, Guido Westerwelle, said Berlin remained strongly opposed to any military intervention in Libya or the use of air strikes against Gaddafi. Westerwelle warned that the consequences of western military intervention were unpredictable and could affect freedom movements in the Arab world.

"Your own instinct is to say 'we have to do something'. But military intervention is to take part in a civil war that could go on for a long time. Germany has a strong friendship with our European partners. But we won't take part in any military operation and I will not send German troops to Libya," he said.

Instead, Westerwelle said there were non-military options that could still be used against Libya, including "targeted sanctions, political pressure and international isolation."

"Considering alternatives to military engagement is not the same as doing nothing," he said. He declined to say what Germany would do before this evening's vote in the UN security council.

A spokesman for David Cameron said: "Clearly there is a race against time at the present time and the situation on the ground is increasingly concerning."

He added: "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."

After weeks of procrastination by the US, Washington backed the resolution after the Arab League joined the calls for a no-fly zone. The Obama administration was stalled by a split between the secretary of state, Hillary Clinton, who favoured a no-fly zone, and the defence secretary, Robert Gates.

The White House, caught in the middle, dithered. Gates, although opposed to the no-fly zone, redeployed US naval vessels close to the Libyan coast and told the president that the military was capable of fighting on a third front. The US crucially insisted that it would only act if there was Arab support in order to avoid it being seen as purely Western intervention. Several Arab countries have promised to provide planes, but insisted on their identity being withheld until the resolution was passed.

It is unclear whether Middle Eastern countries such as Saudi Arabia, the United Arab Emirates and Qatar will participate.

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 said China and Russia have indicated they would abstain. Brazil and India have expressed scepticism about military action but their votes are not needed to secure a majority.

John Kerry, chairman of the Senate foreign affairs committee, speaking before the UN vote, 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."

William Burns, the US under secretary of state, said Washington supported international measures in Libya that are "short of boots on the ground". Burns told the Senate foreign relations committee that a no-fly zone could have "an important, positive, practical" effect, though Washington was still keen to consider other measures.

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