It was not clear when the hostage was killed and his name does not match those of the two Turkish truck drivers kidnapped in Iraq last week.
In the video, found on an Islamic Web site Monday, the Turk identifies himself as Murat Yuce from Ankara and says he worked for the Turkish company Bilin Tur.
In Ankara, a manager for Bilin Tur, Hasan Mecit, told The Associated Press the catering firm has an employee named Murat Yuce in Iraq, but it has no knowledge that he had been killed.
The video shows a man identified as a Turk kneeling in front of three armed men. The man reads a statement in Turkish and says he works for a Turkish company that subcontracted for a Jordanian firm.
"I have a word of advice for any Turk who wants to come to Iraq to work: 'You don't have to holding a gun to be aiding the occupationist United States ... Turkish companies should withdraw from Iraq," he says.
At the end of the statement, the leader of the three presumed kidnappers takes out a pistol and shoots the Turk in the side of the head. The Turk slumps to the ground, and the kidnapper shoots him in the head twice more. Blood is seen on the ground next to his head.
A black banner on the wall behind the kidnappers identfies the group as the Tawhid and Jihad, which is led by the Jordanian militant linked to al-Qaida, Abu Musab al-Zarqawi.
The Turkish truckers association said Monday that it will no longer transport goods to U.S. forces in Iraq in an attempt to get kidnappers to release Turkish hostages.
In other recent developments:
On Saturday, Tawhid and Jihad released a videotape saying it had kidnapped two Turkish truck drivers and threatened to behead them within 48 hours. The tape, which was broadcast on the pan-Arab TV channel Al-Jazeera, showed the men's Turkish passports, identifying them as Abdurrahman Demir and Said Unurlu.
Al-Jazeera said the drivers worked for a Turkish company that delivered goods to U.S. forces in Iraq.
In Ankara, a spokesman for the Turkish Foreign Ministry said it could not confirm the killing of one of the Turkish hostages kidnapped last week in Iraq.
The video carried a date in the Islamic calender - 13-4-1425 - which corresponds with June 1, 2004. It was not clear whether the tape was shot on that date, and released recently, or it was posted on the Internet on that date. ti-sf-jbm
Iraq's top Shiite Muslim cleric on Monday condemned the coordinated attacks on churches in Baghdad and Mosul, saying they "targeted Iraq's unity, stability and independence."
In a statement, Grand Ayatollah Ali al-Husseini al-Sistani described the assaults which killed 11 people and wounded over 50 as "criminal."
Sunday's bombings targeted four churches in Baghdad and one in Mosul during evening services in the first major assault on Iraq's Christian minority since the 15-month-old insurgency began.
"We condemn and reproach these hideous crimes and deem necessary the collaboration of everyone - the government and the people - in putting an end to aggression on Iraqis," said the cleric, who is based in the southern city of Najaf.
"We assert the importance of respecting the rights of Christian civilians and other religious minorities and reaffirm their right to live in their home country Iraq in security and peace."
The unprecedented attacks against Iraq's 750,000-member Christian appeared to confirm community members' fears they might be targeted as suspected collaborators with American forces amid a rising tide of Islamic fundamentalism.
No group claimed responsibility for the assaults.
Many of Iraq's Christians have already fled to neighboring Jordan and Syria to escape violence in the insurgency-wracked nation.