The top general in the region, Gen. John Abizaid, estimated Thursday that insurgent fighters in Iraq total no more than 5,000, and he said the largest and most dangerous groups are Saddam loyalists.
"I would say that this group of Baathists (Saddam loyalists), by far, represents the greatest threat to peace and stability," the general said.
Abizaid, the chief of U.S. Central Command, did not provide details, but said the insurgent forces have considerable training and supplies, plus money from stashes left over from Saddam's rule and from sources outside Iraq "that are not clear to us."
Attacks continued with an attack near Balad, 45 miles north of Baghdad, which killed a U.S. civilian contractor and wounded another American. A roadside bomb blew up as U.S. soldiers tried to defuse it in Baghdad's northwestern neighborhood of Khadra, causing three casualties among the soldiers, a witness said. And in the northern city of Mosul, three soldiers were slightly injured when an improvised explosive device went off near their convoy Thursday night.
Meanwhile, a member of Iraq's Governing Council said the group will study Washington's proposals for a speedier transfer of power but won't necessarily agree with the details.
"On our part, we have our own ideas," said Mahmoud Othman, an independent Kurdish member of the 24-seat body appointed by U.S. administrator L. Paul Bremer four months ago.
Bremer was returning to Baghdad Friday after meeting with President Bush to discuss ways of speeding up the transfer of power to an Iraqi-led government amid deteriorating security in the country.
In other developments:
In other setbacks to U.S. efforts to broaden the force in Iraq, Denmark rejected a push to bolster its 410-member force in the wake of the bombing, and Japan has said that it will almost certainly delay sending a group of non-combat troops to post-war Iraq until sometime next year.
Wednesday's suicide attack killed at least 32 people and injured more than 80.
It was the single deadliest strike against coalition troops since the occupation began in April, and raised fears that Iraqi resistance groups were gradually extending their area of operations to include the country's mainly Shiite Muslim southern regions which have generally been well-disposed toward the U.S.-led coalition.
In Washington, Mr. Bush expressed resolve to curb the violence against coalition forces.
"We're going to prevail," he said. "We've got a good strategy to deal with these killers."
The Bush administration is proposing elections in the first half of next year and formation of a government before a constitution is written, a senior U.S. official said in Washington. In the past, the administration insisted that Iraqi leaders write a constitution and hold elections before the occupying power begins shifting power to Iraqis.
Washington's policy shift is widely seen as part of a response to the worsening security situation and the uprising that already has claimed the lives of more than 50 coalition soldiers this month.
Othman said Bremer would likely meet with members of the governing body on Saturday to present details of the policy shift regarding the transfer of power to a transitional government.
"We will listen to Bremer and he will listen to us," he said.
In Iraq, electric power was back up to pre-war levels, college applications were rising, and, in just two months, Iraqi security forces had been built up to 100,000 from 60,000, Douglas J. Feith, the defense undersecretary for policy, said in a speech Thursday night to the Council on Foreign Relations, a private research group.
But with the U.S. death toll in Iraq approaching 400 and some polls showing increasing criticism of Mr. Bush's handling of post-war Iraq, the administration's aim is to accelerate steps toward an Iraqi takeover and an end to the U.S. occupation.
However, administration officials stress the accelerated handover will not mean U.S. troops are coming out.
"It does not mean we would physically leave the country any sooner," Rumsfeld told troops Thursday in Guam. "What it means is the Iraqis would begin to take on a greater portion of responsibility for governing themselves sooner."
Officials, who spoke to The Associated Press on condition of anonymity, described some of the major guerrilla groups they believe have taken shape in Iraq.
The most significant, The Return Party, is composed primarily of members of Saddam's Baath Party, the officials said. Another, Muhammad's Army, appears to be run by former chiefs of Saddam's security services.
Other threats include Sunni Islamic extremists, some from outside the country, and Shiite extremists who may be receiving support from others in Iran.
The groups are all using classic guerrilla tactics: bombings, snipings and hit-and-run attacks, and at least some are using suicide bombers, officials said.
While the groups have varying degrees of organization and capability, their emergence in the months since Saddam was ousted suggests the U.S. could be facing a sustained insurgency from several fronts.