BAGHDAD, Iraq - Some 800 British troops, accompanied by U.S. Marines, began making their way toward Baghdad today as part of a redeployment ahead of an expected coalition 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.

British Lt. Col. James Cowan said British troops left the southern city of Basra to head for a base north of Hillah, about 60 miles south of Baghdad. Forty U.S. Marines were with them, he said.

"British forces have just started moving this morning into the north of Hillah. They will deploy in that area and will receive their jobs in maintaining security there," he said.

Associated Press Television News footage showed large flatbed trucks carrying armored British vehicles up a road through Iraq's southern desert.

Nearly 800 Scottish soldiers of the First Battalion, Black Watch 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.

The soldiers' families expressed worries today 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.

Prime Minister Tony Blair's decision to grant 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.

In the hostage drama, a video was posted on a militant Web site Tuesday said a Japanese man was kidnapped by Jordanian-born militant Abu Musab al-Zarqawi's group and vowed to kill him within 48 hours unless the demands were met.

Japanese Prime Minister Junichiro Koizumi rejected the demands.

"The Self-Defense Forces will not withdraw," Koizumi told reporters in western Japan. "I cannot allow terrorism and cannot bow to terrorism."