Video posted Tuesday on a militant Islamic Web site showed what it claimed was a Japanese captive kidnapped by Abu Musab al-Zarqawi's (search) group and threatened to behead him within 48 hours unless Japan pulls its troops from Iraq.

Japanese Prime Minister Junichiro Koizumi (search) rejected the demand.

"The Self-Defense Forces will not withdraw," Koizumi was quoted as saying by Japan's Kyodo news agency.

The man, who had long hair and wore a white T-shirt, was identified only as someone connected to the Japanese armed forces. He spoke briefly in halting English and Japanese, addressing himself to Koizumi.

"They asked me why Japanese government broke the law and sent troops to Iraq," the man said in English. "They want Japanese government and Koizumi prime minister, they want to withdraw the Japanese troops from Iraq or cut my head."

He then paused, sighed and switched into Japanese.

"Mr. Koizumi. They seek the withdrawal of Japanese Self-Defense Forces... (and say they) will take my head off," the captive said. "I'm sorry. I want to return to Japan again."

The video's authenticity could not be independently confirmed.

Chief Cabinet Secretary Hiroyuki Hosoda confirmed that Koizumi had instructed the government not to withdraw the troops and said he had been ordered to investigate the facts of the case and to consider measures to free the hostage.

Japan's public broadcaster NHK identified the hostage as Akio Koda of the southern city of Fukuoka, citing Koda's father as saying that the Foreign Ministry had asked him to check whether the video showed his son.

It was the third hostage crisis involving Japanese citizens in Iraq. In April, three Japanese were taken hostage by insurgents who threatened to burn them alive unless Tokyo withdrew its troops from Iraq. Two others were taken hostage separately around the same time. Koizumi refused to meet the hostage-takers' demands, but all five were eventually released unharmed.

The video was being shown repeatedly by NHK and was certain to trigger an uproar in Japan and pose a new test to the government, which has dispatched 500 troops to southern Iraq on a humanitarian mission in support of U.S.-led reconstruction efforts despite strong opposition in public opinion.

Polls show about half of the Japanese oppose the dispatch because of fears it could draw the troops into the fighting there and prompt insurgents and terrorists to target Japanese citizens at home and abroad. Many also argue the dispatch violates the country's pacifist postwar constitution, which limits Japanese troops to self-defense of Japan.

When the captive finished speaking, the video showed him kneeling before three masked militants. One of them read a statement calling the man "an element attached to the Japanese armed forces."

"We give the Japanese government 48 hours to withdraw its troops from Iraq, otherwise his fate will be the same of that his predecessors, Berg and Bigley and other infidels," the man said, referring to the beheadings of British engineer Kenneth Bigley (search) and U.S. businessman Nicholas Berg.

The man's head was bowed to the floor as the militant spoke, and another of the militants grabbed him by the hair to face the camera.

The militant said the Japanese man had entered Iraq through Israel and Jordan. "The documents that prove this will be displayed when his family and the Japanese government identify him," he said.

The video, which lasted just under three minutes, bore the logo of Al Qaeda in Iraq, the new name for al-Zarqawi's group, which was previously known as Tawhid and Jihad. The group has claimed responsibility for the beheadings of Bigley, two American co-workers and Berg, as well as numerous car bombings and other attacks.

Insurgents in Iraq have kidnapped more than 150 foreigners. Most were kidnapped for ransom and freed unharmed, but at least 30 have been killed as part of a campaign to drive out foreign troops and companies.

The new crisis comes at a sensitive time politically for Koizumi. He won a boost in popularity for standing down the hostage-takers last time, but he was weakened in July Parliamentary elections in which the main opposition party made strong gains in the upper house.

Japan's troops also face more hostility in the southern Iraqi city of Samawah, which was chosen for its relative safety. An unexploded mortar was discovered inside the Japanese base there on Saturday, the first time a projectile has been fired into the camp. The mortar, however, was unarmed and nobody was injured.