A European NATO diplomat told AFP that the foreign ministers would gather in the US capital on April 4 to celebrate the signing of the North Atlantic Treaty, often called the Washington Treaty, which laid the foundations for the alliance.

The celebration comes despite lingering concerns among some NATO members about Trump's commitment to the alliance. A gathering of NATO heads of state is expected in late 2019 to mark the anniversary, though details have not been confirmed.

Before taking office he called NATO "obsolete" and soon after a tumultuous summit in July questioned whether the US would honour the alliance's founding principle of mutual defence for newest member Montenegro.

US officials have stressed that Washington is fully committed to NATO and mutual defence, but Trump continues to berate European allies for not spending enough on their militaries, taking particular aim at Germany.

NATO defence ministers will meet at the alliance headquarters in Brussels this week to take stock of progress on agreements reached at the July summit.