Four or five hijackers claiming to be Kurdish rebels seized the ferry off the coast of northwest Turkey late Friday, Transport Minister Binali Yildirim told NTV television.

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

More than eight hours after the attack happened around 1700 GMT, the hostages were still on board the ferry off the coast of Silivri, west of Istanbul.

Television footage showed one person, probably a hijacker, walking around in a ferry corridor and waving from a window.

The hijackers have conveyed demands for food and fuel through the captured captain, said the minister.

Asked if the demands would be fulfilled, Yildirim said, "We'll evaluate them."

The minister said 18 passengers, five of them women, four crew and two interns were aboard the ferry, the Kartepe, which was making its normal route along the northern coast in the Marmara sea.

One of the hijackers claimed to have a bomb and told the ferry's captain that he wanted media publicity, local mayor Ismail Karaosmanoglu told NTV.

Yildirim said the claims could not be verified.

Relatives of the hostages were anxiously awaiting news in the ports of the cities of Izmit and Golcuk, the Anatolia news agency reported.

"We heard about it from the media. We couldn't establish any telephone connections, though we tried many times," a relative of one of the hostages told Anatolia.

Nearby boat and ferry trips between Istanbul and Yalova, about one hour away, were cancelled after the attack, media reports said.

The island of Imrali, where Ocalan, the jailed PKK leader, is being held, lies around 120 kilometres (75 miles) southwest of the hijacking spot. Turkish media say the hijackers may be heading for the island.

"Measures were boosted around the island of Imrali where PKK leader Abdullah Ocalan, who has not been allowed to meet with his lawyers for months, is being held," said the pro-Kurdish Firat news agency.

Patrol boats have been deployed and were scanning a five nautical mile no-go area around the island, it added.

Pro-Kurdish demonstrations are regularly held in Turkish cities in support of Ocalan, who is still considered the PKK's chief despite his imprisonment.

Yildirim said the hijacked Kartepe was being shadowed by coast guard ships which have not had direct contact with the rebels but 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.

Clashes between the PKK and the army have escalated since mid-2011.

A surge of attacks by PKK rebels also caused civilian deaths in Turkey, prompting the Turkish military to launch in October a cross-border operation against rebel hideouts in northern Iraq.

The army operations were mostly concentrated in Turkey's southeast as well as a few areas in the north of Iraq.

Observers say the military action was in response to a domestic outcry but offered no solution to the root of the problem.