Militants loyal to suspected al Qaeda operative Abu Musab al-Zarqawi said on Saturday they had seized three Turkish hostages and would behead them unless Turks stopped working with US-led forces in Iraq.
In the latest attack aimed at derailing the transition to an interim Iraqi government on June 30, guerrillas detonated a car bomb in the town of Hilla, 100 km (62 miles) south of Baghdad, killing 17 Iraqis and wounding 40, the US army said.
The Al Jazeera satellite channel showed footage of the three men said to be Turkish hostages crouching before masked gunmen and holding up their passports. It said it had received the footage and a statement from Zarqawi's Tawhid and Jihad group, threatening to kill the Turkish captives within 72 hours.
Zarqawi's group beheaded a South Korean hostage earlier this week after Seoul rejected a demand to withdraw its forces from Iraq, and last month decapitated a US captive. Both killings were filmed in footage posted on Web sites used by Islamists.
Zarqawi has also claimed responsibility for a series of bloody attacks, most recently a wave of suicide bombings and armed assaults in five cities on Thursday that killed more than 100 Iraqis and three US soldiers.
Turkey is not part of the US-led occupation force in Iraq but many Turkish contractors work as drivers and support staff for American forces. US President George W. Bush is due to attend a NATO meeting in Istanbul on Monday.
Jazeera said the statement it received stated the hostages would be killed unless "Turkish forces and companies that support the occupation forces in Iraq" left by the deadline.
Washington has put a $10 million bounty on Zarqawi's head.
"He remains the number one target inside this country. He is a very effective terrorist," Brigadier General Mark Kimmitt, deputy director of operations for the US military in Iraq, told a news conference in Baghdad on Saturday.
US forces have mounted three "precision strikes" in the rebellious Iraqi city of Falluja over the past week aimed at destroying Zarqawi's safehouses and killing his followers.
Kimmitt said the latest strike, on Friday, may have come close to killing the Jordanian militant. He said several cars were seen driving away from the building after it was hit.
Earlier on Saturday insurgents stormed the offices of two political parties in Baquba, 65 km (40 miles) north of Baghdad.
An attack on the offices of the Supreme Council for Islamic Revolution in Iraq, a Shi'ite group that has been co-operating with the US-led administration, killed three guards and wounded two, officials said.
Kimmitt said six guerrillas were killed in Saturday's fighting in the town. He said one of the dead fighters was found with TNT strapped to his body.