"At a time when Russia has again invaded a sovereign European state, statements that could raise tensions between NATO allies are particularly unhelpful," a State Department spokesperson said when asked about Erdogan's remarks, without naming him.

"The United States continues to encourage our NATO allies to work together to maintain peace and security in the region, and to resolve differences diplomatically."

Erdogan on Saturday told a rally that Greece would pay a "heavy price" for harassing Turkish fighter jets over the Aegean and referenced Turkey's 1922 takeover of the historic Greek city of Smyrna, now Izmir, a bitter memory for Greeks.

Turkey said Greece had used Russian-made air defense system to harass Turkish jets and says Ankara is stationing troops on islands in the Aegean Sea in violation of peace treaties.

Greece rejects the allegations and often accuses Turkey of raising tensions, including through overflights of Greek islands.