Greece's leader said Tuesday he would raise concerns at a NATO summit over North Macedonia's backtracking under a new nationalist government on an agreement over its name.

Greece in 2018 reached an agreement in which its northern neighbor agreed to call itself North Macedonia instead of the historically charged Macedonia, with Athens then lifting its longstanding veto on the former Yugoslav republic entering NATO.

New president Gordana Siljanovska-Davkova at her inauguration in May pointedly called the country Macedonia rather than North Macedonia twice.

Attending a 75th-anniversary summit of NATO in Washington, Greek Prime Minister Kyriakos Mitsotakis said he had "serious concerns" about the new government in Skopje's name usage.

"This is an issue which I do intend to raise," Mitsotakis said at the Council on Foreign Relations, calling the name issue "simply non-negotiable."

"It is not constructive. It does not help the European path of North Macedonia. It's an unnecessary complexity at a time when we should be looking for areas of convergence," he said.

Mitsotakis did not spell out how he would raise the concerns inside NATO. He previously threatened that Greece could jeopardize North Macedonia's path to membership in the European Union, which was already struggling.

The center-right Greek leader noted that a previous left-wing government in Athens reached the agreement to resolve the name dispute.

"I've had issues with the Prespa agreement but I made it very clear that this is an international agreement that binds the country and I respected it -- and I do respect it."