US President Donald Trump voiced frustration with NATO allies for failing to back his war against Iran, deepening transatlantic tensions ahead of a key summit of the military alliance.

Trump has repeatedly criticized the group’s members for failing to spend enough on defense, and frequently suggested he would not adhere to its collective defense commitment.

His latest remarks, made while seated alongside NATO’s secretary-general, came as the alliance’s biggest European members said they would step up their partnership, two weeks ahead of the closely watched summit.

The NATO chief, for his part, sought to assuage Trump with a series of charts underscoring increased European defense spending, before appealing to the American president: NATO allies “want to hear your lead.”