ANKARA, Turkey — A group of Kurdish militants Friday hijacked a ferry with 19 passengers near Istanbul, the country’s transport minister said.

The minister, Binali Yildirim, said “four or five” hijackers claiming to be members of the armed wing of the Kurdish rebel group Kurdistan Workers’ Party, or PKK, commandeered the ship after it set sail from the northwestern port city of Izmit.

The rebels have been fighting for autonomy in Turkey’s mainly Kurdish southeast and have stepped up attacks on Turkish forces in that region in recent months.

One of the hijackers was in the captain’s cabin and was claiming to be carrying a bomb, Yildirim said. The hijackers have so far not made any demands.

Earlier reports had said a lone hijacker had commandeered the boat.

The mayor for the city of Izmit said at least one of the hijackers was armed.

The PKK has increased attacks across the country, killing dozens of Turkish soldiers and civilians. The Turkish military responded by staging an air and ground offensive against rebel hideouts in neighboring Iraq. Turkish police have also detained hundreds of Kurdish activists on suspicion of ties to the rebels.

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

The Hurriyet newspaper’s online edition said security at Imrali was increased. Gunboats were patrolling a five-mile no-go area around the island which is sealed with an electrical fence, it said.

Authorities suspended other ferry services in the Sea of Marmara as a precaution, state-run TRT television reported.

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

TRT television said three coast guard boats were shadowing the ferry.

Yildirim said there were 19 passengers, four crew and two trainees on board.

NTV said the hijackers had collected all the passengers’ mobile phones.

Tens of thousands of people have died since the Kurdish rebels took up arms 1984.

In a previous hijacking by the rebels in 1998, security forces stormed a plane on the tarmac of Ankara airport, and shot and killed a Kurdish rebel hijacker armed with a hand grenade who held 38 people hostage aboard a Turkish Airlines plane. The man was protesting Turkey’s fight against the rebels.