Libya unrest: Cameron vows to rescue stranded Britons
The UK is working "flat out" to get British nationals out of Libya as soon as possible, David Cameron has said.
The prime minister said the situation in Libya was "very dangerous" and the UK's "first priority" was to help the more than 300 Britons stranded there.
A charter flight which should have left for Tripoli on Wednesday was delayed due to a technical problem, the Foreign Office said.
Labour has accused ministers of being "slow off the mark" over Libya.
There are fears of further violence after Libyan leader Col Muammar Gaddafi said enemies of Libya would be executed and vowed to fight to his "last drop of blood" rather than leave the country.
Most of the 3,500 Britons who were living in Libya before the crisis are thought to have already left but some are having difficulty getting out.
Foreign Secretary William Hague has postponed a planned visit to Washington, to remain in London to lead the Foreign Office's response to the unfolding crisis.
British Airways and British Midland International have cancelled flights in and out of the capital Tripoli for the past two days. BA will not operate its daily service on Thursday and BMI has cancelled its flights to and from Libya scheduled for Thursday.
As of 2135 GMT on Wednesday no Foreign Office charter flight had departed the UK or any other destination bound for Libya.
One flight scheduled to leave Gatwick was still grounded with technical problems, a second flight due out of Italy had not yet left. However a plane chartered by BP, with Foreign Office staff on board, has landed in Libya.
The Foreign Office sent a team from Malta to Tripoli airport and they have been registering Britons for the flights, as well as handing out food and water.
Speaking in Qatar, Mr Cameron said diplomats were "working round the clock" to help British nationals out of what was a "very dangerous situation".
"My government is taking every step it can to reach British nationals to make sure they can come out and come home," he said. "That has to be our first priority."
He said he would also support the United Nations Security Council passing a resolution condemning the Libyan regime's actions, saying it would send a "clear warning".
British oil workers stranded in the desert more than two hours from Tripoli have criticised the UK government's speed of response.
"We are living every day in fear of our lives," Jim Coyle told BBC Radio 4's Today programme. "At the end of the day, we are living a nightmare here."
He said the workers had contacted British officials to warn them about the situation but nothing had happened.
"We have asked the British government to come here for days now and they are just totally ignoring us," he said.
"They don't reply to e-mails, they have cut off the phones to Tripoli. Basically we have been left without any protection whatsoever."
Mr Coyle said there was no means of escape as all the vehicles had been looted and local people were heavily armed. He also said the camp - where 300 expatriate workers are based - only had about a day's food left.
Labour have said France, Portugal, Russia and Austria had already managed to get charter aircraft into Tripoli and the UK needed to explain why it had not done so earlier.
"There are hundreds of Britons stranded in Libya at the moment in a daunting, fast-moving and highly dangerous situation," shadow foreign secretary Douglas Alexander said.
"That is why it appears the government has been slow off the mark in its response to those citizens."
Tens of thousands of foreigners are trying to leave the country by land and air and many oil companies are attempting to remove their expatriate staff.