European Union nations on Tuesday unanimously backed a French request for support with military missions in the wake of the Paris attacks, after France invoked a previously unused part of the bloc's treaties to seek help.

France, which launched new airstrikes against Islamic State militants in Syria after Friday's bloodshed, used a little-known article in the EU's Lisbon Treaty which provides for solidarity in the event one of them is attacked.

"Today France demanded the aid and assistance of the whole of Europe. And today the whole of Europe replied in unison 'yes'," Mogherini told a joint press conference with French Defence Minister Jean-Yves Le Drian.

"It is an article that has never been used before in the history of our union."

The EU's Article 42-7 is similar to NATO's Article 5, which the United States activated after the September 11, 2001 attacks to trigger the US-led alliance's intervention in Afghanistan.

The French minister said Paris would now have talks with individual EU nations to see what help they could offer.

"I felt a lot of emotion from my colleagues" over the Paris attacks claimed by the Islamic State group which left 129 people dead, Le Drian said, adding that many of his counterparts had addressed him personally in French to pay their respects.

Britain swiftly said it had offered bilateral aid and would "stand ready to consider any French requests."

In a further show of solidarity, the EU signalled leniency on its tough budget rules after France warned it would not meet its deficit obligations as it steps up security following the attacks.

"The rules of the stability pact do not stop member states from defining their priorities. We understand that the priority is security," EU Economic Affairs Commissioner Pierre Moscovici told a news conference.

"It is in that spirit that we are in discussions with the French government."

- 'Full support' -

France has been seeking international support after the Paris attacks, but with little appetite for invoking the powerful NATO clause it has been seeking other options.

NATO Secretary General Jens Stoltenberg confirmed that France had not requested the use of the alliance's mutual defence clause.

"There has been no request for invoking Article 5. But many NATO allies have offered France support and help. And we are doing so in many different ways, not least by sharing intelligence, working more closely with France in their efforts to fight ISIL," he said.

The official EU request said: "France would like to ask its European partners for their bilateral support in the fight against Daesh (IS) in Iraq and Syria as well as increased military participation from member states in operational theatres where France is deployed."

Mogherini, the former Italian foreign minister who now heads the EU's diplomatic service, said there had been complete unity over France's request.

"Today the EU through the voices of all the member states unanimously expressed its strongest full support and readiness to give the assistance needed," she said.

Brussels would not be involved in any military assistance itself as it does not have a military force of its own, Mogherini said.

But it could offer support in coordinating requests for help by France, which in addition to its strikes on Syria is also involved in fighting jihadists in Mali and in keeping the peace in Central African Republic.

France's Le Drian said the EU's support was a "political act of great significance".

He said it would "allow us in the hours to come to have bilateral talks where necessary" with other EU states to establish what aid France needed.

This aid could either be in support of France's Syria airstrikes but also in other theatres, he said, adding that France "can't be everywhere at the same time."