BAGHDAD, Iraq — Senior American officials are beginning to assemble a new portrait of the insurgency that has continued to inflict casualties on American and Iraqi forces, showing that it has significantly more fighters and far greater financial resources than previously estimated.

Containing that insurgency is the major challenge facing U.S. and Iraqi officials in preparation for Iraqi national elections in January.

Meanwhile, in key developments Thursday:

• The highest-ranking U.S. soldier charged in the Abu Ghraib prison case was sentenced 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 Sgt. Ivan "Chip" Frederick's civilian attorney called the sentence exces-sive.

• The British government agreed to a U.S. request to transfer 850 British troops of the First Battalion, Black Watch Regiment from southern Iraq to an area near Baghdad so U.S. troops could be shifted to insurgent hot spots. The agreement came despite fierce opposition within the governing Labour Party among lawmakers who saw it as a political gift to President Bush ahead of November elections.

• Gunmen ambushed a bus carrying Iraqi women to their jobs at Baghdad International Airport, killing one and wounding 14.

• Three people who worked in Prime Minister Ayad Allawi's office were killed and a fourth was wounded in an ambush in western Baghdad.

• Several mortars fell about two blocks from Allawi's convoy in Mosul, setting off a small blaze and plumes of smoke. No casualties were reported.

• Fallujah leaders called on Allawi's government to pursue a peaceful solution to the military standoff around the city and order a halt to frequent U.S. airstrikes, but fresh clashes erupted between U.S. Marines and insurgents in the insurgent bastion.

New estimates on the resources of the Iraq insurgency contrast sharply with earlier intelligence reports.

When foreign fighters and the network of a Jordanian militant, Abu Musab al-Zarqawi, are counted with home-grown insurgents, the hard-core resistance numbers between 8,000 and 12,000 people, a tally that grows to more than 20,000 when active sympathizers or covert accomplices are included, according to the American officials.

Earlier estimates of the number of insurgents had varied from as few as 2,000 to a maximum of 7,000 fighters. The revised estimate is influencing the military campaign in Iraq but has not prompted a wholesale review of the strategy, officials said.