President Recep Tayyip Erdoğan ordered the Turkish coastguard to stop migrants from making risky Aegean Sea crossings.
Thousands of migrants have massed on the land border with Greece after officials said last week that Turkey would not prevent migrants from leaving for EU territory, sparking violence and an escalating row between Ankara and Brussels.
A video released by the Hellenic Coast Guard shows a Greek Coast Guard vessel being harassed by a Turkish Coast Guard boat.
In the video, the Turkish vessel can be seen maneuvering close to the Greek boat at high speed.
The incident comes amid rising tensions between Greek and Turkish vessels in the Aegean in recent days.
Greece is winning the unorthodox war that has been declared on it by Turkey. The borders are holding; European institutions and EU member-states are standing by the Greeks, confirming that these are Europe's borders; the domestic front is united; even refugees and migrants are coming to realize that they have been had by the cynical Turkish president. In success, however, danger lurks.
The Aegean has been placed in a state of alert as Athens fears that Turkey may, under the guise of migrant search and rescue operations, instigate provocations on Greek islets.
Meanwhile, Turkish President Recep Tayyip Erdogan continued his incendiary rhetoric yesterday, saying that "one day [Greeks] may find themselves in a situation where they will have to seek compassion."