An Irish air force plane has been forced to leave the Libyan capital without evacuating any Irish citizens.
The plane spent four hours at Tripoli airport on Wednesday night but was prevented by Libyan security officials from picking anyone up.
The aircraft has now returned to Valetta in Malta but is expected to go back to Libya to resume the evacuation.
Dozens of British citizens were flown back to the UK on Thursday morning with more flights out of Tripoli due later.
A small number of Irish citizens, said to be less than half a dozen, managed to leave Tripoli on Wednesday night on flights operated by other European governments.
A flight organised by BP and other oil companies has brought home 120 British nationals.
During the night, another airliner, sent by the government, arrived in Tripoli along with an RAF transport plane.
The Foreign Office said "a number of additional planes" could be sent to Libya throughout the day.
It is understood the government has put an SAS unit on standby to help rescue British nationals if required.
Around 3,500 Britons had been living in Libya before the crisis but most of those are thought to have left in recent days.
However, there is particular concern for some British oil workers thought to be stranded in isolated desert camps.
They are struggling to make contact because the phone networks have been disrupted and their supplies of food and water from Libyan cities are running out.
"Some we know have been subjected to attacks and looting. They are in a perilous and frightening situation," Foreign Secretary William Hague said.
After a week of upheaval in Libya, protesters backed by defecting army units are thought to have almost the entire eastern half of Libya under their control.
The country's beleaguered leader Colonel Muammar Gaddafi, who has been in power for 42 years, has vowed to fight to his "last drop of blood" rather than leave the country.
In his first public remarks on the crisis, President Obama has said that the US government is exploring a "full range of options" in response to the crisis.
UK nationals wishing to register an interest in flights out of Libya should call the following numbers: 020 7008 0000 from the UK or 021 3403644/45.
The Foreign Office is advising against all but essential travel to Libya.