Overwhelmed police on the Greek island of Kos beat migrants with truncheons and sprayed them with fire extinguishers on Tuesday as its mayor warned the refugee crisis on the island could end in "bloodshed".

The migrants, mostly Afghans and Syrians, were being relocated to a local football stadium on Kos after camping alongside the island's roads and beaches for weeks.

Greece's police department on Monday said it had suspended an officer serving on the Aegean island of Kos after he was caught on camera allegedly striking a migrant.

"An investigation has been ordered in relation to pictures posted on websites and social media, showing punishable acts by a police officer," the department said in a statement.

Turkey's coastguard rescued 30 Syrians from a boat heading towards Greece after their vessel began to take on water in the Aegean Sea, state media said Tuesday.

The rescue operation came amid a surge in the numbers of migrants seeking to reach Europe, following the Mediterranean's worst migrant disaster in which as many as 800 people drowned off Libya.