LONDON – In a step fraught with risks for Prime Minister Blair, Britain agreed yesterday to send 850 of its soldiers from relatively peaceful southern Iraq to a volatile area near Baghdad, freeing American troops to step up attacks on insurgent strongholds west of the capital.

The move is part of a coalition effort to bring order to Iraq before elections in January. But British lawmakers, many of whom opposed the war, are angry, fearing a major increase in British casualties. And some are grumbling that Britain is “bailing out” President Bush in his bid for re-election.

The Bush administration welcomed the redeployment, with White House spokesman Scott McClellan saying, “We appreciate the contribution,” and State Department spokesman Richard Boucher praising Britain’s key role in the American-led coalition.

Meeting a request from American commanders, the British defense secretary, Geoff Hoon, said an armored battle group from the First Battalion Black Watch would move from its base around the southern port city of Basra into an American-controlled sector close to the capital. Sunni insurgents have been carrying out daily attacks on American troops and Iraqis in the area. The battalion, complete with support units of medics, signalers, and engineers, would stay for a limited period of time, “weeks rather than months,” Mr. Hoon said. Britain’s chief of defense staff, General Sir Michael Walker, later said the deployment would last a maximum of 30 days.

“The government remains totally committed in its support of the interim Iraqi government and the need to hold free elections in January. We also remain committed to protecting innocent Iraqis, to dealing with terrorists, kidnappers, and criminals,” Mr. Hoon told the House of Commons as lawmakers groaned with disapproval.

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

Separately, the highest-ranking American soldier charged in the Abu Ghraib prison case was sentenced yesterday to eight years in prison, the severest punishment so far in the scandal that broke in April with the publication of photos and video showing Americans humiliating and abusing naked Iraqis.

Staff Sergeant Ivan Frederick’s civilian attorney, Gary Myers, called the sentence “excessive” and argued that the military command was at fault for failing to train his client – a veteran military policeman and a corrections officer in civilian life – and for failing to address the horrid conditions at the prison on the western outskirts of Baghdad.

The abuses occurred at a time when American intelligence officers were under strong pressure to gather as much information as possible on the burgeoning insurgency, which threatens the entire American mission in Iraq. Since then, the insurgency has spread throughout Sunni Muslim areas of the country, engulfing regions which were relatively safe for Americans and other Westerners only a few months ago.

Meanwhile, gunmen ambushed a bus carrying Iraqi women to their jobs at Baghdad International Airport, killing one and wounding 14. Also, three people who worked in Prime Minister Allawi’s office were killed and a fourth was wounded in an ambush in Baghdad.