The US failure to co-ordinate with its Nato allies is causing the “brain death” of the military alliance, French President Emmanuel Macron said.
In an extraordinary criticism, Mr Macron said the administration of US President Donald Trump was showing signs of “turning its back on us”.
He was non-committal when asked if he still believed in the concept of collective defence, a cornerstone of Nato’s values.
Last month US troops abruptly began pulling out of north-eastern Syria, resulting in a Turkish incursion into the region against Kurdish forces with whom the West had allied against ISIS.
"What we are experiencing is the brain death of Nato," Mr Macron told The Economist.
“You have no co-ordination whatsoever of strategic decision-making between the United States and its Nato allies. None.
"You have an unco-ordinated, aggressive action by another Nato ally, Turkey, in an area where our interests are at stake."
Mr Trump has previously criticised some Nato states for failing to boost their military spending and is likely to raise it again when the organisation meets early next month in London.
Speaking in Germany, US Secretary of State Mike Pompeo insisted that the Nato alliance remained relevant.
“I think Nato remains an important, critical, perhaps historically one of the most critical, strategic partnerships in all of recorded history,” Mr Pompeo said in Leipzig.
The German Foreign Minister, Heiko Maas, said he did “not believe Nato is brain dead”.
“I firmly believe in international co-operation,” Mr Maas said.
But Mr Macron said Nato would only work 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," he said.
Mr Macron, a committed Europhile, also repeated his support for an EU military force as he warned Europe was on “the edge of a precipice” and risked “in the long run” disappearing “geopolitically” unless it woke up.
He said that Mr Trump “doesn’t share our idea of the European Project”.
“Look at what is happening in the world," Mr Macron said.
"Things that were unthinkable five years ago – to be wearing ourselves out over Brexit, to have Europe finding it so difficult to move forward, to have an American ally turning its back on us so quickly on strategic issues.
"Nobody would have believed this was possible."
Mr Macron’s comments were quickly rebuked by his close ally, German Chancellor Angela Merkel.
"I don't think that such sweeping judgments are necessary, even if we have problems and need to pull together," Mrs Merkel said.