The comment, which will increase concerns that the Atlantic allies are headed for a bust up at their summit next month, was reported Thursday by US news site Axios and confirmed by a European diplomat.
The G7 summit in Quebec earlier this month was already known to have been a frosty affair, with Trump facing off alone against the leaders of the world's next six richest democracies over trade.
But his barbed aside underlines that the US leader's hostility to multilateral agreement extends to the NATO alliance, the cornerstone of Western security for decades.
News of his remark came just as the White House and the Kremlin announced that Trump and NATO's great foe President Vladimir Putin of Russia will meet in Finland straight after the alliance's summit.
The NATO allies are due to meet on July 11-12 in Brussels for a summit that will be clouded by Trump's angry claims that Europe is exploiting the United States by underspending on defense.
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