Rebels in Libya are retreating from their former strongholds along the eastern coast as they come under fire from Col Muammar Gaddafi's forces.

The rebels have now lost the key oil port of Ras Lanuf and the nearby town of Bin Jawad, and are also in full retreat from Brega.

In the west, the rebel-held town of Misrata is still coming under attack from pro-Gaddafi troops, reports say.

Britain's prime minister said arming rebels had not been ruled out.

David Cameron's remarks echoed those made earlier by US President Barack Obama.

The BBC's Ben Brown, in Ajdabiya, said that the rebels are essentially now in retreat as they simply cannot compete with the discipline and fire-power of Col Gaddafi's forces. He says the current situation is a dramatic about-turn for the rebels who, over the weekend, had seized a string of towns along the coast and seemed to be making good progress.

One rebel on the edge of Brega was angered that the coalition appeared to have slowed down its campaign.

"We want two things: that the planes drop bombs on Gaddafi's tanks and heavy artillery, and that they give us weapons so we can fight," 27-year-old fighter Younus Abdelghaim told AFP news agency.

An unconfirmed Reuters report suggests civilians are already leaving Ajdabiya, heading east by car towards the relative safety of the rebel-held city of Benghazi.

France and the US say they are sending envoys to Benghazi to liaise with the interim administration there. And British Foreign Secretary William Hague said a UK diplomat met Libyan rebel leaders in Benghazi earlier this week. The UK has also made moves to expel five Libyan diplomats based in London, Mr Hague told MPs on Wednesday.

Mr Obama told reporters on Tuesday that Col Gaddafi had been greatly weakened by the coalition air strikes and would ultimately step down.

Mr Cameron said that the RAF had flown 24 sorties over Libya on Tuesday night which had destroyed artillery and an armoured vehicle near the strategic city of Sirte.

Sirte is Col Gaddafi's birthplace and lies on Libya's northern coast, roughly half way between the capital Tripoli and Benghazi.

On Tuesday, the Obama administration and British foreign secretary suggested the UN resolution authorising international action in Libya could also permit the supply of weapons.

This message was reinforced by Mr Cameron in parliament on Wednesday.

"UN [Security Council Resolution] 1973 allows all necessary measures to protect civilians and civilian-populated areas, and our view is this would not necessarily rule out the provision of assistance to those protecting civilians in certain circumstances," he said.

"We do not rule it out, but we have not taken the decision to do so."

But Russian Foreign Minister Sergei Lavrov disputed that UN resolution 1973 gave a mandate to arm the rebels.

"The Nato Secretary General Fogh Rasmussen declared that the operation in Libya was being staged to protect the population and not to arm it - and here, we completely agree with the Nato secretary general," he said.

Several thousand people have been killed and thousands wounded since the uprising against Col Gaddafi's rule began more than six weeks ago.