No-fly zone or sanctions among options being considered as world bids to force Libyan leader to end the violence

International efforts to respond to the Libyan crisis are gathering pace under US leadership after a still defiant Muammar Gaddafi launched counterattacks to defend Tripoli against the popular uprising now consolidating its hold on the liberated east of the country.

The White House said Barack Obama planned to call David Cameron and France's president, Nicolas Sarkozy, to discuss possible actions, including a no-fly zone or sanctions to force the Libyan leader to end the violence. Switzerland said it had frozen Gaddafi's assets.

Gaddafi, in power for 42 years, has used aircraft, tanks and foreign mercenaries in eight days of violence that has killed hundreds in the bloodiest of the uprisings to shake the Arab world. Up to 2,000 people may have died, it was claimed by a senior French human rights official.

But there was no sign Gaddafi was prepared to change course. In another semi-coherent and abusive speech on Thursday, he accused protesters of being drugged and agents of al-Qaida. "Their ages are 17. They give them pills at night, they put hallucinatory pills in their drinks, their milk, their coffee, their Nescafé," he said in a telephone interview with Libyan state TV – suggesting he may already have left his heavily guarded Tripoli compound.

It only boosted the growing impression that he is desperate and out of touch with reality. "This is the speech of a dead man," said Said el-Gareeny in the eastern city of Benghazi, which is now in opposition hands.

"People always warn about al-Qaida and say this will become an Islamic state ... to get support from western countries. This isn't true. The Libyan people are free. That's it."

Cameron will take personal charge of efforts to set up convoys, protected by the military, able to evacuate British and other citizens stranded in camps in the Libyan desert amid growing fears that they could be taken hostage. The Foreign Office estimates there are 150 Britons, mostly oil workers and support staff, stranded in remote and isolated camps scattered over a large distance.

A possible airlift by special forces will also be examined. The defence secretary, Liam Fox, said he was co-ordinating a response with Nato as well as looking at the state of Libyan air defences and the risk they pose to UK forces. British special forces are in Malta, with some reports that they are in Tripoli.

Heavy fighting was reported from the important town of al-Zawiya, 35 miles west of Tripoli, while armoured units commanded by Gaddafi's son Khamis and other loyalist forces were deployed eastwards along the coastal road towards Misurata, the country's third largest city and a major port – said to be in the hands of rebels who are now equipped with heavy weapons.

Reports from Libya said between 23 and 100 people had been killed in al-Zawiya, which controls the western approaches to Tripoli.

Medical sources in the capital reported that the corpses of those killed in recent days and injured patients were removed from the Tripoli medical centre and another hospital.

Witnesses said they had been taken to Mitiga military airport. "They are trying to hide the evidence and cleaning up the streets and telling people to go to work," said one man. "But from dusk onwards it's a ghost town."

In eastern Libya, many soldiers have now withdrawn from active service and some are supporting the revolt, with a former Gaddafi minister helping to organise the next stage of the uprising.