Washington DC - President Donald Trump will attend the NATO summit in Turkey in July, Secretary of State Marco Rubio said Wednesday, amid lingering questions about Washington's commitment to the security alliance.

"The President himself will be attending the next NATO meeting of heads of state, where all these points will be made clear," Rubio told the House Foreign Affairs Committee, referring to the July 7-8 conference in Ankara. "We're still in NATO, but NATO needs significant changes."

Rubio described the summit, which will bring together 32 member states, as "probably the most important meeting in NATO's history, because there's some things that need to be cleared up and fixed."

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Relations with the military alliance have remained strained since European governments declined to join the US and Israel in their war against Iran. Washington has withdrawn some troops from Europe.

In separate testimony to Congress on Tuesday, Rubio vented frustration over Spain's refusal to let US forces use its bases for strikes against Iran.

"We have members of that alliance that are basically denying the use of those bases in a contingency," Rubio said, adding that was cause "to question the entire thing."