The BBC has been given a video showing Turkish coast guards using sticks against a boat full of migrants as they sail to Greece in the Aegean Sea.
The incident is said to have happened in Turkish waters as the migrants were on their way to the island of Lesbos.
The migrants accused coast guards of attacking them, but the coast guards say they were trying to stop the boat without harming the occupants.
The EU and Turkey are discussing new moves to curb the flow of refugees.
Europe is facing its biggest refugee crisis since World War Two. Last year, more than a million people entered the EU illegally by boat, mainly going from Turkey to Greece.
Most of them were Syrian, fleeing the country’s four-year civil war. Another 2.7 million Syrian refugees are currently in Turkey, which is struggling to cope with the influx.
More than 132,000 migrants have arrived by boat into Greece so far this year – a large increase on the same period last year.
The volunteer in Lesbos who sent the video to the BBC said the incident occurred in Turkish waters opposite the island.
The 30-second clip shows people wearing life jackets on a rubber dinghy being hit with poles by men standing on another vessel. The people being hit shout out in Farsi and Arabic.
The boat of migrants eventually moved away and made it to Lesbos. A Turkish coast guard official later confirmed the incident, but said the officers had been trying to stop the engine “without harming the refugees”.
He said the standard procedure was to tie migrant vessels to a coast guard boat and bring it back to Turkey, “but this boat of refugees didn’t stop the engine and continued on to Greece,” he added.
Our correspondent says he has heard of similar incidents involving other migrants.