Attention has increased on Article 42.7 of the EU's governing treaty -- meant to help any member state that comes under attack -- as Trump has deepened questions over whether the United States would help defend NATO allies.

Momentum grew further after a drone struck a British base on Cyprus, which currently holds the EU's rotating presidency, at the start of the Middle East war in March.

Representatives from the EU's 27 nations in Brussels will stage an exercise in May simulating how the bloc would respond in case of an armed attack, a senior EU official said.

"What we're doing is that we're looking and going through the practical ways: How does it work? What can we do?" the official said.

The exercise -- which will be followed up by another one run by EU ministers -- comes as Trump has rocked faith in NATO.

The US leader has angrily lashed out at European countries over their response to his war with Iran and suggested he could quit the 77-year-old military alliance.

European Commission head Ursula von der Leyen at the Munich Security Conference in February said that the bloc needed to bring its own "mutual defence clause to life".

But EU officials insist that they do not see Article 42.7 as a substitute for NATO's Article Five collective defence clause.

Twenty three of the EU's member states are also in NATO and they are keen not to fuel any thinking by Trump that he can walk away from the alliance.

"It is not a mutual defence clause. It is a clause of mutual assistance," the senior EU official said. "It's not Article Five".

The EU's Article 42.7 has only been invoked once -- by France after the 2015 terror attacks in Paris -- and there remain deep questions over what it entails.

The clause says that if the territory of an EU country is attacked then the other members states "shall have towards it an obligation of aid and assistance by all the means in their power".

But it leaves it up to each country to decide on the sort of assistance it offers and underscores that NATO remains the "foundation" of collective defence for most EU members.

When France triggered the article in 2015 it asked for contributions to help its fight against terrorism, allowing it to bolster its operation to protect key sites.