"We think they are four or five... They say they belong to a branch of the terrorist organisation," he told ATV television station, referring to the military command of the banned Kurdistan Workers' Party (PKK).

He said that the hijackers had made no demands so far.

Yildirim said that 19 passagers and four crew were aboard the ferry, the Kartepe, which was on its normal route in the sea of Marmara between Izmit and Golcuk.

He said the hijackers had ordered the ferry to head to the southwest, adding that it had enough fuel to continue for 120 nautical miles (220 kilometres).

The island of Imrali, where jailed PKK leader Abdullah Ocalan is being held, lies in the sea of Marmara around 120 kilometres to the southwest of the point where the ferry was hijacked. Turkish media say the hijackers may be aiming for the island.

Yildirim said that the hijacked ferry was being shadowed by coast guard ships, saying they had not had direct contact with the rebels, but had managed brief exchanges with the captain.

The PKK listed as a terrorist organisation by Turkey and much of the international community took up arms for Kurdish independence in southeastern Turkey in 1984, sparking a conflict that has claimed some 45,000 lives.