The declaration, made during a visit to Greece, comes as US President Donald Trump is questioning the North Atlantic military alliance, repeatedly calling it a "paper tiger" for not helping out in his war against Iran.

The mutual defence clause enshrined in Article 42.7 of the Treaty of the European Union, which requires other EU countries to come to the defence of any member under attack, is "steadfast", Macron said.

It is unambiguous and, "in substance, stronger than Article 5" in NATO's founding treaty, he said in a news conference alongside Greek Prime Minister Kyriakos Mitsotakis.

"It allows solidarity between member countries but leaves no option."

The United States's commitment to backing Europe's defence has been shaken under Trump.

The US leader has sought closer ties with Russia and reduced military support for Ukraine, while questioning the validity of NATO's common defence mechanism.

Macron has been co-leading international conferences on military cooperation alongside British Prime Minister Keir Starmer outside of NATO.

He has also been promoting the idea of France's armed forces taking a greater role helping its EU defence partners, including through deployments of nuclear warheads on French warplanes.