The main source of concern for the United States is the Sunni Triangle northwest of Baghdad, a stronghold of the minority Sunni Muslim community that held power under former president Saddam Hussein.

More than 80 US soldiers and some 700 Iraqis, according to the US military, have been killed in fighting in Iraq over the past 12 days, much of it around Fallujah, west of Baghdad.

US-led coalition forces have also faced fierce fighting from some Shiite Muslims in the south, where the situation has stabilized in recent days, according to US military officials.

The Mehdi Army militia of radical Shiite cleric Moqtada Sadr, who has been branded an outlaw by the coalition, is now only a limited threat, military officials said.

US troops have massed around the holy city of Najaf in a bid to snare Sadr who is wanted on charges in connection with the murder of a rival cleric last year. The US military has said its mission is to "capture of kill" Sadr.

The coalition also wants to disband his militia of 3,000-6,000 poorly equipped and trained fighters.

Washington wants to isolate Sadr within the majority Shiite population, that was repressed under Saddam and that hopes to benefit from the handover of power to Iraqis on June 30.

"Sadr continues to marginalize himself from the rest of the Shiites," General John Sattler, Central Command's chief of operations, told reporters in Washington from the command's regional headquarters in Doha.

"I think we can say he has had his day in the sun," Sattler said.

Sattler acknowledged more coordination has been seen among the fighters, with the Marines reporting an increasingly sophisticated resistance in Fallujah.

"I can't identify their numbers or who they are, but the enemy we are fighting is tough," he said. "We have noticed a little more coordination than in the past, but that's to be expected over time."

General Richard Myers, chairman of the Joint Chiefs of Staff, warned Thursday that future military operations will have to take place in Fallujah and its surrounding area, where foreign fighters have mixed with members of Saddam's deposed regime.

The Pentagon has dubbed these fighters "dead-enders" -- combatants who have nothing lose because they expect nothing from a democratic Iraq.

The United States is also worried by the influx of foreigners joining arms against the coalition in the Sunni region.

"We know for a fact that a lot of them for sure are coming through Syria ... and some from the Iranian border," Myers said. Sattler said Marines have increased patrols along the Syrian border.

Analysts warn against the US military's relative optimism.

"The most fundamental problem is the persistent resistance in the Sunni Triangle, but also the growing restlessness in the Shiite south," said Loren Thompson, who oversees security studies at the Lexington Institute, a think tank in Arlington, Virginia, near Washington.

US military officials are "caught between conflicting strategies," Thompson said.

They would need to reduce troops in the Shiite regions, where they are seen more and more as "irritant," to boost forces in the Sunni areas, he said.

Juan Cole, an Iraq specialist at the University of Michigan, said a prolonged US occupation could lead to alliances between Iraq's diverse communities.

"Iraqi nationalism should not be underestimated," he said.

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