Turkish President Recep Tayyip Erdogan has said he no longer recognises the leader of neighbouring Greece and will refuse to meet him at a planned summit.
Mr Erdogan accused Greek Prime Minister Kyriakos Mitsotakis of seeking to block the sale of American-made F-16 fighter jets to Turkey while visiting the US.
"There's no longer anyone called Mitsotakis in my book," Mr Erdogan told reporters after a cabinet meeting.
Greece said it would not get into a "confrontation of statements".
The row comes amid Turkey's threats to block a proposed expansion of Nato.
Ankara has accused Finland and Sweden of harbouring members of Kurdish militias that it says pose a risk to Turkey.
Mr Erdogan also accused Greece, a Nato ally, of harbouring "terrorists" and said that allowing Athens' readmission to the security bloc in 1980 had been a mistake.
Tensions have been growing between the Mediterranean neighbours after Mr Mitsotakis urged a joint session of the US Congress not to drop a ban on Turkey's membership of a purchasing programme for the next generation F-35 fighter jet last week.
The Biden administration has reportedly been considering dropping the ban, imposed after Ankara purchased an S-400 air defence system from Russia in 2019.
Mr Erdogan said this amounted to lobbying US officials against Turkey and violated an agreement "not to involve third countries" in disputes between Ankara and Athens.
The Turkish President also announced a planned summit with Mr Mitsotakis later this year had been cancelled, pledging to "never agree to have a meeting" with the Greek leader.
While Greece and Turkey are nominally Nato allies, the two have a tense relationship that has regularly threatened to break out into open conflict.
They have been at odds over the status of Cyprus since the 1974 conflict that divided the island in two and in 2020 Ankara deployed warships to explore for gas reserves in waters recognised internationally as Greek.