Up to five gunmen identifying themselves as members of a branch of the outlawed Kurdistan Workers' Party (PKK) have hijacked a ferry off the coast of the western Turkish city of Izmit with 19 people on board.

Binali Yildirim, the Turkish transport minister, told NTV television on Friday that one of the hijackers is in the captain's cabin and is claiming to be carrying a bomb.

"We think they are four or five... They say they belong to a branch of the terrorist organisation," he told ATV television station, naming it as the military command of the PKK.

He added that the hijackers have not yet made any demands.

The PKK have been fighting for autonomy in Turkey's mainly Kurdish southeast and have recently stepped up attacks on Turkish forces in that region.

The Turkish military has responded by staging an air and ground offensive against PKK hideouts in neighbouring Iraq.

Turkish police have also detained hundreds of Kurdish activists on suspicion of ties to the fighters.

The pro-Kurdish Firat news agency, without citing sources, said the ferry was allegedly heading towards the heavily guarded prison island of Imrali, where the Kurdish group's chief Abdullah Ocalan is serving life in prison.

The PKK and Kurdish politicians have been calling for Ocalan's release as a condition for peace.

State-run TRT television reported that no raid was planned for the safety of the passengers, and that coast guard boats were shadowing the ferry as it was sailing off the coast of Izmit in the Sea of Marmara.

Source: Agencies