A rebel Iraqi group is reported to be holding three Japanese nationals hostage.
The group said it would kill the two men and a woman unless Japan withdraws its forces from the region.
The Japanese government is reported to have said it had no plan to pull troops out of Iraq and demanded the immediate release of the hostages.
Meanwhile, US-led coalition soldiers across Iraq are reported to be engaged in clashes with Sunni and Shia militants.
A US military convoy was attacked west of Baghdad - hours after American troops fought street battles with both groups in parts of the Iraqi capital.
Fighting between Sunni rebels and US marines has continued for a fourth day in Falluja; hospital officials there said about 300 people have been killed.
The commander of American troops in Iraq has said the coalition forces are no longer in control of the cities of Najaf and Kut where Shi'ite militiamen have seized key buildings.
Lieutenant-General Ricardo Sanchez said operations would continue across Iraq until the insurgents were defeated.
Rumsfeld admits 'serious' Iraq problems
Last night, the US Defence Secretary, Donald Rumsfeld, admitted that US forces are encountering serious problems in Iraq, but stressed that insurgents were relatively few in number.
Speaking to reporters at the Pentagon, Mr Rumsfeld denied the military situation was spiralling out of control.
British civilian kidnap
Earlier this week, a British civilian was kidnapped in Nasiriyah, the scene of heavy fighting between radical Shi'ite militiamen and Italian troops.
Officials named the man as Gary Teeley, a British contractor.
A Foreign Office official in London confirmed that Mr Teeley was missing, but would not say what he was doing in Iraq or comment on the manner of his disappearance.
