There were several reports that the two men were Iranians.
However, a relative who managed to contact a passenger on his mobile telephone said on Turkey's NTV television that the hijackers were Turkish and had claimed to be members of al-Qaeda.
Yousef al-Sharif, Al Jazeera's correspondent in Ankara, Turkey's capital, said: "Turkish security forces are in talks with the hijackers. Passengers who escaped said the men were not Turkish and were speaking Arabic or Farsi.
"The passengers said the kidnappers claimed they were from al-Qaeda and were protesting against US foreign policy. They are demanding to be flown to a Middle East country.
"The men are holding four hostages, two crew members and two passengers."