The west was edging towards a possible military confrontation with Muammar Gaddafi's regime, as the US deployed naval and air force units around Libya, and David Cameron ordered contingency plans for Britain to help enforce a no-fly zone.

The steps were part of a concerted western effort to hasten Gaddafi's downfall and avoid a prolonged civil war and a humanitarian crisis on Europe's southern flank.

The US froze $30bn in assets held by Gaddafi and his officials, a record for the United States. France meanwhile dispatched two aircraft full of medical and humanitarian supplies to the rebel-held town of Benghazi, the start of what it said would be "a massive operation of humanitarian support for the populations of liberated territories."

Cameron said he had told the Ministry of Defence and the chief of the defence staff to draw up plans for a no-fly zone in coordination with Britain's Nato allies and report back to him within days.

A no-fly zone would be designed principally to prevent attacks on Libyan people by the Gaddafi regime – mainly by his helicopter gun ships.

Cameron suggested the UK might even consider arming the Libyan opposition forces if Tripoli used more violence to crush demonstrations.

Officials said discussions on a range of military options began last week between British and US officials at the Pentagon. They said that the support of US and British armed forces might also be required to protect corridors to channel humanitarian relief into Libya through Tunisia and Egypt, if further conflict brought about a mass displacement of the population and a collapse in the food supply.

The prime minister discussed imposing a no-fly zone over Libya in a telephone call with president Nicolas Sarkozy of France. An emergency summit of all the EU's 27 leaders is now expected to be held in Brussels next week.

The military deployment and heightened rhetoric coming from Washington and London were designed to multiply the pressure on Gaddafi and his top officials, but there are serious political obstacles.

Western officials say any military intervention in the unfolding conflict would have to be coordinated by Nato and would require the approval of the UN Security Council, and that is far from guaranteed. Russia and China, who both hold a veto, have voiced their opposition to any outside interference. France too has cautioned about Nato involvement.

Two army officers from Benghazi's military committee confirmed jets that had taken off from Tripoli each dropped bombs on the city at around 4pm.

"We had several anti-aircraft positions, which fired at the planes and they left," said Colonel Hamid Belkhair, who runs the rebel military in Benghazi. "Then they fled."

Officials in Benghazi's new interim council, which aims to provide a political face to the revolution, said Gaddafi loyalists intended to cut water and electricity supplies to cities in the east and attack them from the air.

The city of Misrata, in western Libya was also bombed and strafed by helicopters, residents told the Guardian.

An anti-aircraft position was established on Benghazi's waterfront against an expected increase in attacks. The east of the country remains lightly armed compared with the loyalist army.

The rebel military has no anti-aircraft missiles and only a small number of old jets. The remnants of the air force still loyal to the veteran dictator are flying predominantly Russian-made MIGs.

The Pentagon's announcement that it was repositioning naval and air forces in the region gave no details of what units were involved. "We have planners working and various contingency plans and ... as part of that we're repositioning forces to be able to provide for that flexibility once decisions are made," said Colonel David Lapan, a Pentagon spokesman.

In Geneva, the US secretary of state, Hillary Clinton, said that the warships could be used for humanitarian and rescue missions. "There is not any pending military action involving US naval vessels," she said.