BASRA, Oct 27: Hundreds of British soldiers left southern Iraq on a risky mission closer to Baghdad on Wednesday to free up US forces for a possible assault on Fallujah , as Tokyo stood firm after militants threatened to behead a Japanese hostage.

Interim Prime Minister Iyad Allawi has warned of escalating violence in the runup to elections promised by January, but has pledged to restore stability - by force if necessary.

To this end, Britain agreed last week to a controversial US request to move 850 crack troops from the southern city of Basra to the restive province of Babil, just south of Baghdad, as US soldiers focus on wresting the restive hub of Fallujah from militant control.

"The convoy has begun to leave," said a British army spokesman.

The redeployment marks the first time since the invasion of Iraq that British troops have left southern Iraq for US-controlled areas, where they are considered more likely to come under hostile fire.

Scores of tanks on transporters, along with other army vehicles, formed several convoys accompanied by US vehicles.

Helicopters were seen buzzing overhead as the battle group, led by the Black Watch battalion, rumbled northwards for a limited deployment, though Britain's Prime Minister Tony Blair left open the possibility that other British troops may have to replace them.

"The Black Watch will come back by Christmas. As to what then happens, we can't be sure at the moment," he said.

"We don't believe that there will be a further requirement for our troops but I can't commit myself, I can't guarantee that," Mr Blair told parliament in London.

The British deployment closer to Baghdad includes up to 550 Black Watch soldiers, coupled with specialists from other units, including engineering and communications.

JAPAN FIRM: In a parallel hostage drama, Japan insisted it would not bow to the demands of a militant group linked to Iraq's most wanted man, Abu Mussab al Zarqawi, that threatened on Tuesday to behead a 24-year-old Japanese traveller unless Tokyo withdrew its 600 or so troops within two days.