France has launched a military force with other countries including Britain outside the framework of the European Union, as Paris tries to keep London close to European defences after Brexit.

After months of negotiations with Germany, which France also wants at the centre of the force, the so-called European Intervention Initiative aims to bring together a coalition of willing militaries ready to react to crises near Europe's borders without NATO or the United States.

France's Defence Minister Florence Parly held a ceremony in Luxembourg at which Germany, Belgium, Britain, Denmark, Estonia, the Netherlands, Spain and Portugal signed a letter of intent.

"Our conviction ... is that Europeans should be strong, capable of ever more, able to protect ourselves and our sovereignty," Ms Parly told reporters after the ceremony.

French President Emmanuel Macron proposed the idea in a speech last September, initially facing scepticism because the European Union in December also agreed a landmark defence pact that aims to develop forces and weapons together.

Italy was also set to be involved, Ms Parly said, but dropped out following the election of a new government of anti-establishment left and far right parties.

Despite concerns about potentially overlapping European defence initiatives, NATO Secretary-General Jens Stoltenberg welcomed the decision, saying it would help modernise European militaries and make them quicker to mobilise.

"I believe it can strengthen the readiness of forces, because we need high readiness," Mr Stoltenberg, said following a joint meeting of EU defence and foreign ministers in Luxembourg.

European governments, pilloried by US President Donald Trump for slashing military spending after the Cold War, face the challenge of remedying long-running problems with helicopters and jets that are grounded for lack of parts.

Ms Parly also said the French-led initiative should go some way to answering Trump's call for Europe to do more on security.

EU defence ministers agreed today the rules of that pact, known as Permanent Structured Cooperation (PESCO), which will be backed by a new multi-billion euro defence fund from 2021.

That will not include Britain because it is leaving the European Union at the end of March next year, but London hopes to be involved in some defence projects as a non-EU nation.

Tánaiste and Minister for Foreign Affairs Simon Coveney was in Luxembourg to attend the meeting of EU foreign ministers.

In an earlier statement, Mr Coveney said Ireland has "played an active role in shaping the Common Security and Defence Policy to equip the EU to act as an effective international peace provider in support of the UN.

France now sees itself at the centre of evolving defence structures after Brexit, including the EU defence pact, with a bigger role for Spain, and Mr Macron's intervention force that involves Britain, Europe's biggest military along with France.

Britain's role in the force appears to have helped overcome differences between Germany, which has a historical resistance to military missions using force, and France, which wants quicker reaction to crises such as in Mali in 2012, where Paris intervened to stop Islamist militants taking over.

Berlin wanted Mr Macron's force within PESCO, which includes all EU countries except for Britain, Malta and Denmark.

France insisted it be outside the more deliberate PESCO structures that are long term, focused on developing weapons and other gear.

German Foreign Minister Heiko Mass appeared to share France's impatience with slow-moving EU decision-making, saying Berlin wanted EU foreign policy to decided by a majority vote in future, not based on unanimity, which allows countries to block.

Despite its departure, Britain seeks a security treaty with the EU by 2019, eager to keep access to EU databases, arms deals and share intelligence, and many EU countries support the idea.