Volunteer fighters that came to Iraq from other Arab nations are resisting with a zeal outlasting that of their Iraqi counterparts, posing a threat to U.S. troops and to Iraq's stability.

U.S. officials say the bulk of them are from Syria, but coalition troops also have encountered guerrilla resistance from Sudanese, Egyptians and Jordanians.

U.S. Brig. Gen. Vincent Brooks said Monday that foreign fighters appeared to be "well armed" and are still a threat despite the fact that coalition troops control most of Iraq. He also criticized them for prolonging the war-weary misery of normal Iraqis and destabilizing the country.

"We find that these volunteers claim that they are here in Iraq to protect the Iraqi people," Brooks said. "In fact, they put the people at risk and contribute to instability, and there's no role for them in the future of Iraq."

Thousands such fighters flocked to Iraq before its government imploded, inspired in part by Arab pride, glossy recruiting pamphlets and passionate pleas from terrorist mastermind Osama bin Laden.

How many are in Iraq is uncertain. But as Washington phases out its combat missions and Iraqi soldiers give up and go home, the foreigners appear to be fighting on.

"There are pockets of foreigners in Iraq who have decided to fight to their last breath," U.S. Gen. Tommy Franks said over the weekend.

"Until we have a sense that we have all of that under control, then we probably will not characterize the initial military phase as having been completed and the regime totally gone," he said.

On Saturday, a man with Syrian identity papers shot and killed a U.S. Marine at a checkpoint in Baghdad. And U.S. commanders say hundreds of foreign fighters were killed at a recent shootout at the Imam Mosque in Baghdad. A U.S. Marine also was killed and 20 were wounded.

U.S. military officials discovered about 300 suicide vests lined with explosives hidden inside a school last week. On Monday, they said 80 more vests appeared to be missing and could be in the hands of foreign fighters.

"Where we do find them, they're well armed," Brooks said. "We think that some of the explosive vests were meant for them."

Franks described the foreign fighters as mercenaries paid by the Iraqi government. He said they were recruited through pamphlets passed out in neighboring countries to work as "everything from suicide bombers to small group hit squads."

Over the weekend, U.S. forces stopped a bus with 59 men of military age carrying $650,000 in cash and a letter offering rewards for killing American soldiers. Military officials did not say where they were from, but said the bus was headed for Syria.

The Bush administration has accused Damascus of harboring leaders of the old Iraqi regime and supporting terrorism. Secretary of State Colin Powell even raised the possibility of diplomatic and economic sanctions. Syria has repeatedly denied the charges.

Brooks said foreign fighters were coming in from Syria in the "the greatest density" but he could not say if they were sponsored by the government.

Complicating the mix are dissident groups operating in the Iran-Iraq border region.

The Tehran-based Supreme Council of the Islamic Revolution in Iraq claims 10,000 fighters and says many are already sprinkled throughout Iraq in anticipation of Saddam's fall.

It is one of Iraq's largest opposition groups but has refused to partake in the U.S.-led civil administration, saying it smacks of colonialism.

Meanwhile, the People's Mujahedeen, an Iranian dissident group based in Iraq, claims that Iranian forces have taken advantage of the chaos in Baghdad to make cross border raids on its group. It said 10 of its fighters were killed Saturday by Iran's revolutionary guards and intelligence forces.

Washington considers the People's Mujahedeen a terrorist group.