"The Greek Prime Minister is seeking help from the U.S. Against what? Against Türkiye. Whatever you do, we will continue to and are ready to do whatever is necessary," the president told a conference marking the week of the Prophet Muhammad's birth in the capital Ankara.
Erdogan also repeated his criticism of Greek pushbacks of irregular migrants in the Aegean and Mediterranean seas.
"We watch in awe as Greek ferries bury innocent people in the Aegean and Mediterranean," the president said, as he also criticized the world for remaining silent in face of Greek violations.
His criticism came a day after Greek officials said Mitsotakis would agree to meet with Erdogan on the sidelines of a European summit later this week if the latter asked.
Long-simmering tensions between the two countries, which are NATO allies but historic foes, have flared anew recently due to treaty violations and provocations by Greece.
Erdogan has accused Greece of "crimes against humanity," referring to its treatment of migrants and illegal pushbacks, and of arming islands in the Aegean Sea that have a demilitarized status.