Britain has joined growing calls for the possible imposition of no-fly zone over Libya in an effort to prevent further attacks by Colonel Muammar Gaddafi on his own people.
David Cameron told the Commons he had ordered the Chief of Defence Staff, Sir David Richards, to begin work with key UK allies on plans for a military no-fly zone.
The Prime Minister said the international community could not tolerate Col Gaddafi's "illegitimate" regime using military force on the civilian population and he warned that they needed to be ready to act if the repression worsened.
"We do not in any way rule out the use of military assets. We must not tolerate this regime using military force against its own people," he told MPs. In that context I have asked the Ministry of Defence and the Chief of the Defence Staff to work with our allies on plans for a military no-fly zone."
US Secretary of State Hillary Clinton, attending the United Nations Human Rights Council in Geneva, said that America was also actively looking at the possibility of a no-fly zone and, in Washington, the Pentagon said it was moving some US armed forces in the region nearer to Libya in case they were needed, but did not go into details.
Mr Cameron said: "For the future of Libya and its people, Col Gaddafi's regime must end and he must leave," he said. "My message to Col Gaddafi is simple: go now." But the Libyan leader told the American ABC network: "All my people love me. They would die to protect me."
The Prime Minister told the House that 600 British nationals had been brought out, including almost 100 picked up from the desert in a series of daring rescue missions over the weekend by RAF Hercules aircraft.
While he praised the efforts of the military and diplomatic personnel involved, he accepted that there were "lessons to be learned" over how the evacuation was handled. Labour leader Ed Miliband said the Prime Minister should apologise for the Government's slow initial response.
Downing Street said Mr Cameron had spoken to French President Nicolas Sarkozy by telephone.
"They agreed that the actions of the Libyan regime had been totally unacceptable," a Number 10 spokesman said. "They agreed that British and French experts should work together on the range of possible options for increasing pressure on the regime."