"At the NATO summit a year ago, he did not explicitly support the idea in place since the North Atlantic Treaty that when one country of the alliance is attacked, the others will come to their aid," Florence Parly said on France 2 television.

"That casts a certain doubt" over the alliance formed by the treaty in 1949, she said.

Trump, who once derided the alliance as "obsolete", says the US shoulders too much of NATO's costs and has insisted that European members must increase their spending.

At the coming NATO summit in Brussels on July 11 and 12, "I think the US president will insist very strongly that its allies, in particular in Europe, pay the burden which the US says it is bearing," Parly said.

"That's why building a common European defence is necessary, in this situation where we don't really know if the assumptions we've lived with for the past 70 years are still valid," she said.

France and Germany have already taken a "historic" step by agreeing to jointly build the next generation of fighter jet and tank for their forces, and to form an integrated rapid response force, she said.

"Many European countries still buy their military equipment mainly from non-European countries -- I don't need to cite the United States -- and that is also something we need to change," she added.