In a videotape sent to Aljazeera on Saturday, the three captives were shown kneeling in front of two masked gunmen while holding up their passports. The captives said their names and explained in Turkish the nature of their work in Iraq.

They also asked the Turkish people to demonstrate against Bush's upcoming visit to Turkey and to push for the pull-out of Turkish companies working with US forces.

The group, al-Tawhid and al-Jihad, had also claimed responsibility for beheading American businessman Nicholas Berg in May and South Korean translator Kim Sun-il this week. 

The tape was released as President George Bush was travelling to Turkey for a NATO summit which has on its agenda the approval of military training assistance for the new Iraqi government. 

Unpopular war

The Iraq war and Bush are deeply unpopular in Turkey, the only predominantly Muslim country in NATO. The deaths of the three captives during the alliance meeting would further inflame passions already high because of opposition to the war. 

Turkish officials in Baghdad said they believed the workers were captured two days ago, but that the circumstances were not immediately clear. 

"We don't have any information right now," said a Turkish consular official, who asked to be identified only by his surname, Gungor. "We are still waiting." 

Seven Turks working for companies doing business with the US were kidnapped near Falluja but were freed this month after their employer said "prominent families" in the area secured their freedom.