Greece has complained to Turkey in a border dispute linked to a riverbed change, a foreign ministry source said Friday.
The ministry official confirmed an earlier report by state agency ANA, saying Athens had made a protest to Ankara.
Foreign Minister Nikos Dendias on Wednesday said there was a "dispute" over the "exact boundary" of the Evros, the river known as Maritsa in Turkey that divides the two countries.
"The riverbed has changed," Dendias said.
Greek reports on Friday said Turkish troops had occupied a strip of land that is normally submerged at this time of year, and is on Greece's side of the border.
The main opposition Syriza party on Friday demanded an immediate briefing "over Turkey's arbitrary and aggressive actions".
Dendias was criticised in parliament for saying on Wednesday that "all these things can be resolved via a joint committee and the relevant measurements."
"We're talking about a few tens of metres," he said.
There was tension in the area in March after Turkey encouraged tens of thousands of asylum-seekers to travel to the border with Greece in the hope of crossing into the European Union.
For days, there were skirmishes on the border as migrants trying to break through threw stones at Greek riot police who fired tear gas at them.
Turkish police also bombarded Greek forces with tear gas at regular intervals, and Athens accused Turkish police of handing out wire cutters to migrants to help them break through the fence.
Greek border forces have also claimed that Turkish troops have recently fired shots into the air.
Ankara accused Athens of beating migrants and firing live rounds at them, alleging that some died of bullet injuries.
The Greek government categorically denied using undue force.