Japan says the apparent kidnapping of one of its nationals in Iraq will not affect its troop deployments there.
Militants said their captive was badly injured in the ambush
Japan's foreign ministry said Akihiko Saito - described as an employee of a British security firm - was reported missing after his car was ambushed.
Several others travelling with him are thought to have been killed.
The militant group Ansar al-Sunna issued a statement on an Islamist website saying it had seized Mr Saito near Hit, west of Baghdad.
The group also posted pictures of his passport.
The militants said Mr Saito, 44, had been injured in an ambush that had killed four other foreign contractors and 12 Iraqis.
Ansar-al-Sunna has claimed responsibility for numerous kidnappings in the past, and released videos of previous attacks on Iraqi government forces.
Since receiving information about the kidnapping from the British security firm Hart, the Japanese government has set up a special task force to try to secure Mr Saito's release.
"We are hurrying to confirm Mr Saito's whereabouts, and if his reported seizure or serious injury turns out to be true, we must move urgently," Foreign Minister Nobutaka Machimura told reporters.
"For now, our top priority is to gather accurate information."
But Defence Minister Yoshinori Ohno told reporters in Tokyo that the kidnapping "won't affect the activities of [Japan's] Self-Defence Forces" in Iraq.
Mr Ohno added that the security situation in the southern Iraqi city of Samawa - where some 550 Japanese troops are providing humanitarian help - "has not worsened".
In October 2004, an Islamic militant group beheaded Japanese backpacker Shosei Koda, who was abducted while visiting Baghdad.
Five other Japanese were taken hostage in April 2004, but were later freed unharmed.
Mr Saito's case is very different from that of the other six, says the BBC correspondent in Tokyo, Jonathan Head.
These hostages were criticised by many people in Japan for endangering their lives and causing problems for the government.
Given the job he was doing, Mr Saito is likely to get even less sympathy, our correspondent says.
Australia also faces a hostage crisis in Iraq. Last week insurgents seized Douglas Wood, a 63-year-old engineer, and demanded that Australian troops leave Iraq by Tuesday.
Australia has appealed for Mr Wood's release, but Prime Minister John Howard ruled out any negotiations with his captors.
No new information on Mr Wood's fate has emerged, despite the passing of the deadline set by his kidnappers for troops to leave the country.