Britain is working with its allies on a plan to establish a military no-fly zone over Libya, says David Cameron.
The prime minister said the threat of "further appalling steps" being taken by Col Muammar Gaddafi to oppress his own people was behind the talks.
He said he did not rule out "the use of military assets" in Libya and said the "murderous regime" must end.
Fewer than 150 British citizens are thought to remain in Libya and only a "very small proportion" want to leave.
The government would continue to do "all we can" to get them out, he said.
Libya has been embroiled in turmoil as protesters demand Colonel Gaddafi - the Middle East's longest serving ruler - steps down.
The UN estimates that about 100,000 people have fled anti-government unrest over the past week and thousands may have been killed or injured in a violent crackdown by the regime.
In a statement to MPs after returning from a tour of the Middle East, Mr Cameron said the government was "taking every possible step to isolate the Gaddafi regime".
The UK has frozen Col Gaddafi's British-held assets and those of his family, and withdrawn their diplomatic immunity and an export ban has been imposed on Libyan banknotes, which are printed in Britain.
Mr Cameron said that Britain had secured a European Union agreement on freezing the assets of a "wider group of individuals" connected to Col Gaddafi and banning them from entering the EU. A wider arms embargo was also being imposed against Libya.
The PM told MPs there would be "further isolation of the regime by expelling it from international organisations" and further use of asset freezes and travel bans to encourage those "on the fringes of the regime, that now is the time to desert it".
He added: "And 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.
"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."
The Labour MP Ann Clwyd told him that a no-fly zone could "save thousands of lives if he's [Col Gaddafi] going to bomb his own people from the air".
Mr Cameron said they would comply with international law but planning for a no-fly zone had to start now because no-one knew what Col Gaddafi would do to his own people and one might have to be put in place "very quickly".
But he added that trying to secure a no-fly zone over a country as large as Libya was "not without its difficulties": "We would be trying to cover a vast area, it would take a serious amount of military assets to achieve it."
But he added: "I do think it's one thing we need to look at, look at it urgently and plan for, in case we find, as we may well do, that Col Gaddafi is taking further appalling steps to oppress his people and that is why the conversations are taking place today."
'Day of reckoning'
The Ministry of Defence said that the plans were still at an early stage and the initial focus would be on which countries would back it and what military assets they could then be deploy to enforce it.
The BBC's Jonathan Beale said it was not yet clear what contribution Britain might make but one source had suggested RAF Typhoon fighter jets could be stationed at a UK base in Cyprus, although the source made clear no decisions had yet been taken.
But Lord West, the former head of the Navy, said no-one should underestimate the difficulties of implementing a no-fly zone, starting with ensuring there was a UN resolution to approve it.
"There's the complexity of making sure you've got the aircraft - and that's controlling-type aircraft like AWACS aircraft, fighters and things like that - to actually put it in place, so it's a meaningful no-fly ban.
"There's the issue of do you give authority to shoot down fixed-wing aircraft, shoot down helicopters? Do you give authority to your planes that if they're illuminated by fire-controlled radar from surface-to-air missiles, that they're entitled to go and hit those surface-to-air missiles?
"There are quite a lot of complexities of rules of engagement and just the whole military operation."
Mr Cameron also said the UK was acting to prevent a "humanitarian crisis" and would be flying in tents and blankets on Tuesday, and had dispatched technical teams to help migrant workers get home.
'Chaos and incompetence'
But Labour leader Ed Miliband urged him to apologise for the government's handling of the crisis last week - when a rescue effort for Britains in Libya was delayed - saying they had been let down by "chaos and incompetence".
However, he welcomed the comments that Britain's defence chiefs had been asked to work with Britain's allies on plans for a military no-fly zone and he welcomed the increasing international isolation placed on the regime.
Meanwhile, Foreign Secretary William Hague has called for an immediate end to violence against anti-government demonstrators in Libya and warned Col Gaddafi's supporters that there will be a "day of reckoning" for anyone involved in human rights abuses.
Addressing a meeting of the United Nations Human Rights Council in Geneva, he said: "This is a warning to anyone contemplating the abuse of human rights in Libya or any other country: Stay your hand. There will be a day of reckoning and the reach of international justice can be long.
"We must now maintain the momentum we have attained to ensure that there can be no impunity for crimes committed in Libya and to help bring about an immediate end to the violence."