© Yves Herman/ReutersFrance's President Nicolas Sarkozy talks with David Cameron at the start of an EU emergency summit on Libya and north Africa in Brussels.

Nicolas Sarkozy has called for targeted air strikes against Muammar Gaddafi's regime if his forces use chemical weapons or launch air strikes against civilians.

As the EU foreign policy chief, Lady Ashton, warned that a no-fly zone could risk civilian lives in Libya, the French president told an emergency EU summit in Brussels that air strikes may soon be justified.

"The strikes would be solely of a defensive nature if Mr Gaddafi makes use of chemical weapons or air strikes against non-violent protesters," Sarkozy said. The French president qualified his remarks by saying he had many reservations about military intervention in Libya "because Arab revolutions belong to Arabs".

Sarkozy said he had won the support of David Cameron for his plan which would have to be approved by the UN, Arab states and Libyan opposition groups.

Downing Street sources declined to be drawn on his remarks. But Sarkozy appears to have set out the key events - use of chemical weapons or air strikes against civilians - that would give legal cover for military action.

In a joint letter issued on the eve of Friday's summit, Sarkozy and Cameron warned that Gaddafi might be guilty of crimes against humanity. This could give Britain and France the necessary legal cover under the Geneva conventions for a no-fly zone or targeted air strikes.

The tough rhetoric from Sarkoy came as Cameron faced his first major public confrontation with Ashton as she used the EU emergency summit to dismiss Anglo-French calls for a no-fly zone over Libya.

Emboldened by unease among Nato members for the proposal, Ashton has been telling the EU's 27 leaders that a no-fly zone would be highly risky and could end up killing large numbers of civilians.

One EU diplomat said: "The risks are high for potential civilian casualties and potential collateral damage. The efficiency of a no-fly zone is very questionable. Apart from anything else, European command and control facilities would not be able to get a no-fly zone up and running in less than five or six weeks, and Nato is suggesting it would take at least three to four weeks.

"The question is whether, in political terms, a no-fly zone can achieve what you want it to achieve."

Downing Street declined to be drawn on Ashton's decision to set herself so forcefully against one of Cameron's central proposals for responding to the Libyan crisis.

One British source said: "We proposed planning for a no-fly zone so early because it will take some time. That work has not been completed."

Ashton will argue that it is her role to act as the voice of all 27 EU member states. Cameron and Sarzkoy, who were the driving forces behind the emergency summit, appear to be ahead of most leaders.

As he arrived at the summit Cameron said Europe must seize the moment to send a clear signal to Gaddafi that his behaviour was unacceptable and that he must stand down.

The prime minister said: "It's important that the countries of Europe show political will, show ambition and show unity in being clear that Colonel Gaddafi must go, that his regime is illegitimate and what he is doing to his people is completely unacceptable.

"We are going to step up today in the European council measures to isolate that regime and measures to put pressure on that regime and we should also plan for every eventuality as I've been arguing now for around two weeks. This is absolutely vital work.

"But at the same time I think it is the moment for Europe to understand we should show real ambition about recognising that what's happening in north Africa is a democratic awakening and we should be encouraging these countries down a democratic path.

"It's a moment for Europe to say what we've done in the past hasn't always worked. Now we should be reaching out to these countries, offering them a new partnership, opening up our markets and welcoming their approach of greater democracy, greater freedom, greater human rights. This is a potentially good moment for our world and we should grab it and seize it and try and shape it."

British officials were playing down prospects of a major breakthrough at the summit. The main focus will be to toughen the arms embargo against Libya and co-ordinate European efforts if a humanitarian disaster is declared in Libya. Britain is also pressing for a reform of the EU's £1bn "neighbourhood policy" which provides support to countries in north Africa and the Middle East.

The tough Anglo-French rhetoric has been met with deep scepticism across the EU. Sarkozy's unilateral decision to recognise Libya's transitional council as the legitimate representative of the Libyan people was seen as grossly premature. "Sarkozy is being irresponsible," one EU diplomat said.

Mark Rutte, the Dutch prime minister, said: "I find it a crazy move by France. To jump ahead and say 'I will recognise a transitional government' in the face of any diplomatic practice, is not the solution for Libya."

Cameron's tough language about the need for Europe will be seen as a signal that the EU should avoid a repeat of its failures when Yugoslavia collapsed into civil war in 1991. It took American intervention - the negotiations at Dayton in 1995 and in the Kosovo military action in 1999 - before the war was finally brought to an end.