Updated

Nearly 800 British forces left their base in southern Iraq on Wednesday, heading north toward Baghdad (search) to replace U.S. troops who are expected to take part in an offensive against insurgent strongholds.

The deployment came hours after Iraq's most feared militant group released a video threatening to behead a Japanese captive within 48 hours unless Japan withdraws its troops from Iraq. Japan's prime minister, a staunch U.S. ally in Iraq, took a tough stance and rejected any troop withdrawal.

Large flatbed trucks were seen carrying armored British vehicles up a road through Iraq's southern desert as the redeployment began for the nearly 800 Scottish soldiers of the First Battalion, Black Watch, headquartered in Basra.

"We can confirm that there is some movement," said British military spokesman Maj. Charlie Mayo. He gave no specifics on troop numbers, citing security concerns.

British officials have been vague on the precise destination, though the troops are expected to deploy in an area just south of Baghdad. Some media reports indicated the Black Watch would be sent to Iskandariyah, a town that sees frequent violence between the capital and the city of Hillah.

The soldiers' families expressed worries Wednesday that the redeployment puts the troops in greater danger.

"It wasn't a cake walk in Basra but it's going to be a lot, lot more dangerous up there," said James Buchanan, 56, from Arbroath in central Scotland, who has two sons with the regiment in Iraq. "They're going to get one hell of a kicking this time," he said.

The troops are to replace U.S. forces who are expected to take part in offensives against insurgent strongholds west and north of the capital in an attempt to bring order to Iraq before elections in January.

The American military wants the British to assume security responsibility in areas close to Baghdad, so U.S. Marines and soldiers can be shifted to insurgency strongholds west of the capital, including Fallujah.

Prime Minister Tony Blair's decision to agree to the U.S. request for redeployment is a politically sensitive one for the British leader, whose popularity has plummeted because of his support for the Iraq war.

Britain's 8,500 troops are based around the southern port city of Basra in a relatively peaceful area of Iraq. Sixty-eight British soldiers have been killed in Iraq, compared with more than 1,000 U.S. troops.

The political pressure mounted with last week's kidnapping of British aid worker Margaret Hassan, who heads CARE International's operations in Iraq. Hassan, 59, who also holds Iraqi and Irish citizenship, was kidnapped on her way to work in Baghdad. No group has claimed responsibility.

Interim Iraqi Prime Minister Ayad Allawi said Tuesday that more extremists are massing in Fallujah and warned of increasing terrorist attacks to come. On Saturday, insurgents ambushed and executed about 50 unarmed Iraqi soldiers as they were heading home from a U.S. military training camp northeast of Baghdad.

On Wednesday, a motorcycle bomber attacked a U.S. convoy in central Iraq, killing one American soldier and wounding another, the U.S. military said in a statement. The name of the soldier killed was being withheld pending notification of next of kin.

In the hostage drama, a video of the Japanese captive was posted on a militant Web site Tuesday, and the Al Qaeda-linked militant group led by Jordanian-born militant Abu Musab al-Zarqawi vowed to kill the hostage within 48 hours unless the 500 Japanese troops in Iraq leave the country.

Japanese Prime Minister Junichiro Koizumi rejected the demands.

"The Self-Defense Forces will not withdraw," he said. "I cannot allow terrorism and cannot bow to terrorism."

The captive, who in the video had long hair and wore a white T-shirt, was identified by the Japanese government as 24-year-old Shosei Koda. 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.

Tokyo has dispatched some 500 troops to the southern Iraqi city of Samawah on a humanitarian mission to purify water and rebuild schools in support of U.S.-led reconstruction efforts.

The video claimed that Koda was connected to the Japanese military, but the government denied that. Friends and relatives suggested Koda had gone to Iraq as a tourist.

"Mr. Koda is a private individual who is not related to the Self-Defense Forces or the government of Japan," Foreign Minister Nobutaka Machimura said.

When the captive finished speaking, the video showed him kneeling before three masked militants. 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.

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

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 and has allied itself with Usama bin Laden. 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.

The United States has offered a $25 million bounty for the capture or killing of al-Zarqawi, who is believed to be hiding in the militant stronghold of Fallujah.