A Turkish navy patrol boat on Friday fired shots in Greek waters in the southeast of the Aegean Sea, Greek military staff said, qualifying the incident as "serious".
The patrol boat left the area, off the island of Farmakonisi, after a Greek frigate intervened by sending warnings, a military source told AFP on condition of anonymity.
"This was a serious, qualitatively different incident" to what Greece denounces as recurring Turkish violations of its sovereign areas in the Aegean, he said.
The Greek navy remained in a state of "reinforced surveillance" on Friday after the incident, he added.
According to the source, Turkey had on Thursday night issued a notice of manoeuvres in the area, which were rejected by Greece because they "engaged a zone in Greek waters".
In late January, against a backdrop of increased tensions between the two neighbours, Greek and Turkish warships were involved in a brief faceoff near a group of disputed Greek islets in the Aegean.
Located just off the Turkish coast and claimed by Ankara, the uninhabited rocky Imia islets are a historic flarepoint in a long-running demarcation dispute.
That incident came after the Greek Supreme Court blocked the extradition of eight former Turkish army officers who had fled to Greece after the failed July 15 coup.
Turkey lashed the ruling as "political" and threatened to scrap a "readmission agreement" under which Turkey has been taking in migrants landing illegally in Greece.