Suspected supporters of an al-Qaeda leader are threatening to behead three Turks taken hostage in Iraq, according to a report on al-Jazeera TV.
The gunmen set an ultimatum in the videoed message
Video footage aired by the Qatar-based station showed the three men surrounded by masked militants claiming to be linked to Abu Musab al-Zarqawi.
In an accompanying message they said Turkish firms must withdraw from Iraq within 72 hours or the men will die.
They also urged Turks to protest during the Nato summit in Istanbul.
The militants said Turkish citizens should stage demonstrations against the presence of US President George W Bush, who is already in Turkey in preparation for the meeting.
Top of the agenda for Mr Bush will be seeking the Nato alliance's help to stabilise Iraq.
A Turkish consular official in Baghdad, speaking on condition of anonymity, told the Associated Press that the three hostages had disappeared two days ago, but declined to give any further information.
Observers say the abductions are likely to fuel the already high anti-war sentiment in Turkey. The Nato member was opposed to the war and its troops are not part of the US-led occupation force in Iraq
However, many Turkish contractors work as drivers and support staff for US forces there.
In Seoul there have been vigils for a murdered South Korean hostage
Al-Jazeera said the statement it received with the tape said the hostages
would be killed unless "Turkish forces and companies that support the occupation forces in Iraq" left by the deadline.
In the video shown by the Qatar-based TV station the three Turks were kneeling in front of two black-clad gunmen, holding up their Turkish passports.
On the wall behind the group was a black banner bearing the words "Tawhid and Jihad", the name of Zarqawi's organisation.
Zarqawi's movement has already claimed responsibility for the beheading of two other hostages captured in Iraq - US man Nick Berg and South Korean hostage Kim Sun-il.
Both killings were filmed and the footage posted on Islamist websites.
The body of the murdered South Korean arrived in his hometown of Busan on Saturday.
About 6,000 people took part in a candlelight vigil in the South Korean capital, Seoul, in memory of the 33-year-old translator whose execution earlier this week shocked the nation and the world.
Mr Kim was working for a security company supplying the US military when he was abducted.
South Korea rejected demands from the kidnappers that it cancel plans to deploy 3,000 troops to northern Iraq.
Mr Kim's body was found on the road between Baghdad and Falluja on 22 June.
The hostage's family accused the government of betrayal for failing to secure his release.