French President Emmanuel Macron, in an interview with British weekly The Economist, warned fellow European countries that they could no longer rely on the United States to defend North Atlantic Treaty Organization (NATO) allies.
"What we are currently experiencing is the brain death of NATO," Macron was quoted as saying.
Asked whether he still believed in the Article Five "collective defence" stipulations of NATO's founding treaty - under which an attack against one ally is considered as an attack against all allies - Macron answered: "I don't know."
"(NATO) only works if the guarantor of last resort functions as such. I'd argue that we should reassess the reality of what NATO is in the light of the commitment of the United States," Macron added.
The United States is showing signs of "turning its back on us", as demonstrated by President Donald Trump's sudden decision last month to pull troops out of northeastern Syria last month without consulting the allies, the French leader said.
That move caught NATO's leading European powers - France, Britain and Germany - by surprise and paved the way for Turkey, another NATO member, to launch a cross-border military operation targeting Syrian Kurdish forces.
At the time Macron decried NATO's inability to react to Turkey's offensive and said it was time Europe stopped acting like a junior ally of the United States when it came to the Middle East.
The European allies fear the U.S. withdrawal from northeastern Syria will cause a security vacuum that can be exploited by Islamist militants.
France has long pressed for closer European defence cooperation but has faced resistance from Britain and others which say the United States remains key to Western defence, especially in the face of a more assertive Russia.
Trump has been strongly critical of European countries' heavy reliance on the United States for their defence and the failure of some, notably Germany, to hit a NATO target of spending 2% of national output on defence.