The Foreign Office has been organising the evacuation of British nationals from Libya after Colonel Muammar Gaddafi vowed to fight to his "last drop of blood".
The Libyan dictator sparked fears that violent reprisals against protesters could escalate still further as he called on his supporters to take to the streets and "attack them in their lairs".
In a defiant and rambling address on state television, he promised to die a martyr rather than leave the country he has ruled for 41 years.
With no sign of an end to the bloody clashes, Foreign Secretary William Hague said he was stepping up efforts to help British citizens leave the country. He is seeking landing clearance for a charter flight to arrive within the next 48 hours and a Royal Navy frigate is being re-deployed to waters near Libya.
Mr Hague said: "The safety of British nationals in Libya is of paramount concern to us. In light of the fluid and dangerous situation, we are urgently reinforcing our team on the ground with specialist personnel to provide help and assistance to British nationals.
"Already this week British nationals seeking to leave Libya have encountered significant difficulties. Many are currently in Tripoli airport without immediate flights out of the country following flight cancellations, closures of airspace and so on."
Mr Hague said that, as well as working with airlines to help as many British nationals leave as possible, the Foreign Office was sending a charter plane to Libya in the next 48 hours.
"We are urgently seeking landing clearances and permissions from the Libyan government," the Foreign Secretary added.
HMS Cumberland is also being redeployed from the eastern Mediterranean to international waters near Libya, he said, so that it is in a position to "play a role in assisting British nationals".
Some British Embassy staff and families who are not needed to deal with the current situation in Libya are leaving while Foreign Office personnel trained in crisis response are being sent out.