Internal NATO relations have become increasingly strained following Turkey’s apparent refusal to allow Sweden and Finland into the fold, with fellow NATO member Greece becoming the latest European nation on Turkish President Tayyip Erdogan’s chopping block.

Erdogan took a swing at Greek Prime Minister Kyriakos Mitsotakis this week when he accused him of attempting to block a U.S. arms sale of F-16 fighter jets to Ankara.

“There’s no longer anyone called Mitsotakis in my book,” he told reporters following a Monday cabinet meeting.

The Turkish president also said he would refuse to meet his Greek counterpart for a previously planned summit later this year.

Erdogan’s comments came a week after the Greek prime minister met with U.S. lawmakers on Capitol Hill and urged them to consider NATO’s security when making “defense procurement decisions concerning the eastern Mediterranean.”

“We are always open to dialogue. But there is only one framework we can use to resolve our differences – international law and the unwritten principle of good neighborly relations,” Mitsotakis told U.S. lawmakers. “The last thing that NATO needs at a time when our focus is on helping Ukraine defeat Russia’s aggression is another source of instability on NATO’s southeastern flank.”

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The Greek prime minister did not mention Erdogan or neighboring Ankara, but his comments alluded to a long-standing spat with Turkey over alleged airspace violations.

Turkey and Greece, both of whom are NATO members, have shared a complex relationship for more than a century. But Athens and Ankara’s latest tiff amid Russia’s aggression in Europe could spell trouble for the very military alliance that Russian President Vladimir Putin would like to see dismantled.

“All nations act in their own self-interest, all the time,” Michael Ryan, former deputy assistant secretary of defense for Europe and NATO, told Fox News. “[Erdogan] defines the Turkish self-interests and he defines how they pursue it. And in this case, he views Turkey as a rising regional power, and he is pushing hard in every direction to certain Turkish prerogatives.”

Read more: Fox News