Libyan rebels are struggling to take advantage of the aerial shield provided by international forces protecting them against most of Muammar Gaddafi's heavy weaponry.

Government forces shelled rebels regrouping in the dunes outside a key eastern city, while their snipers and tanks roamed the last major opposition-held city in the west.

The military air campaign appears to have hobbled Gaddafi's air defences and artillery and rescued the rebels from impending defeat.

But the opposition force, with more enthusiasm than discipline, has not exploited the gains.

The international alliance, too, has shown fractures as officials struggle to articulate an endgame.

China and Russia, which abstained from the UN Security Council vote authorising the intervention, called for a ceasefire today, after a night when strikes hit Tripoli, destroying a military seaport in the capital.

Most of eastern Libya is in rebel hands but their forces have struggled to make headway on the ground.

Ajdabiya, city of 140,000 that is the gateway to the east, has been under siege for a week. Outside the city, a ragtag band of hundreds of fighters milled about today, clutching mortars, grenades and assault rifles.

Some men clambered up pylons in the rolling sand dunes of the desert, squinting as they tried to see Gaddafi's forces inside the city. The group periodically came under artillery attacks, some men scattering and others holding their ground.

Since the uprising began on February 15, the opposition has been made up of disparate groups even as it took control of the entire east of the country. Regular army units that joined the rebellion have proven stronger and more organised, but only a few units have joined the battles while many have stayed behind as officers try to co-ordinate a force with often antiquated, limited equipment.

The rebels pushed into the west of the country in recent weeks, only to fall back to their eastern strongholds in the face of Gaddafi's superior firepower.

Misrata, Libya's third-largest city and the last major western redoubt for the rebels, was being bombarded by Gaddafi's forces today, his tanks and snipers controlling the streets, according to a doctor there who said civilians were surviving on dwindling supplies of food and water, desperately in search of shelter.

"Snipers are everywhere in Misrata, shooting anyone who walks by while the world is still watching," he said. "The situation is going from bad to worse. We can do nothing but wait. Sometimes we depend on one meal per day."

Mokhtar Ali, a Libyan dissident in exile elsewhere in the Middle East, said he was in touch with his father in Misrata and described increasingly dire conditions.

"Residents live on canned food and rainwater tanks," he said, adding that Gaddafi's brigades storm residential areas knowing that they will not be bombed there. "People live in total darkness in terms of communications and electricity."

But while the airstrikes can stop Gaddafi's troops from attacking rebel cities - in line with the UN mandate to protect civilians - the United States, at least, appeared deeply reluctant to go beyond that toward actively helping the rebel cause to remove him.

US Defence Secretary Robert Gates and others said the US military's role will lessen in coming days as other countries take on more missions and the need declines for large-scale offensive action like the barrage of Tomahawk cruise missiles fired mainly by US ships and submarines off Libya's coast.

One senior defence official said that the attacks so far had reduced Libya's air defence capabilities by more than 50%.

That has enabled the coalition to focus more on extending the no-fly zone, which is now mainly over the coastal waters off Libya and around Benghazi in the east, across the country to the Tripoli area this week.

France has proposed that a new political steering committee outside Nato be responsible for overseeing military operations.

French Foreign Minister Alain Juppe said it would bring together foreign ministers of participating states - such as Britain, France and the United States - as well as the Arab League. It is expected to meet soon either in Brussels, London or Paris.