Under close American scrutiny, a small team of U.N. nuclear experts arrived in Baghdad on Friday to begin a damage assessment at Iraq's largest nuclear facility, left unguarded by U.S. troops during the early days of the war and then pillaged by villagers.

The arrival of the team -- whose members are not weapons inspectors -- marked the first time since the Iraq war began that representatives from the International Atomic Energy Agency (search), the United Nations' nuclear agency, returned to the country. The IAEA had long monitored Saddam Hussein's nuclear program.

Iraqi scientists who have surveyed the damage at the Tuwaitha (search) plant said looters left behind piles of uranium (search) and spilled radioactive materials. The scientists cemented over the spilled materials to prevent leakage or further exposure to residents in the area.

The United States tried to keep the IAEA out of postwar Iraq. But it reluctantly agreed to allow the agency's return under pressure from the arms-control community, which was concerned about Tuwaitha's safety and U.S. capability to secure the area and account for its contents.

"The IAEA can best tell what's missing, and they're fully prepared to do that pretty rapidly," said David Albright, an American nuclear expert.

U.S. military commanders acknowledged this week that, after nearly three months on the ground, they remain unequipped to handle the nuclear site.

"I know that the Tuwaitha facility is larger than the assets we have now in country to deal with it," said Lt. Gen. David McKiernan, commander of U.S. ground forces in Iraq.

For more than a decade, the IAEA monitored nearly two tons of uranium and radioactive materials tagged at the defunct facility. But the United States cut U.N. inspectors out of the weapons hunt when it went to war without U.N. backing.

For this trip, the Pentagon limited the number of IAEA staff to seven and said the assessment would have to be completed within two weeks.

The team was originally told it would have to stay at the site in tents set up by the army, but the IAEA said Washington had since agreed to let the team stay at the U.N. compound in Baghdad. On Friday, IAEA spokeswoman Melissa Fleming said the plans had changed again and the team would be staying at the American-controlled al-Rasheed Hotel (search).

Pentagon officials, who spoke on condition of anonymity, said the IAEA team would be accompanied at all times by American troops and weapons experts. But Fleming said the team would work independently.

"We're not going to conduct any activities with the military," she said.

The Pentagon has also stressed that the IAEA visit would be a one-time event to enforce the nuclear Nonproliferation Treaty -- and not a weapons inspection that might set a precedent for future U.N. searches for weapons of mass destruction in Iraq.

Team leader Brian Rens, who arrived in Kuwait on Thursday, said the IAEA mission is to "determine what has been lost and any other material which is in an unsafe condition, to repack it to the extent possible, secure it, verify it and seal the building."

Rens said the team's mission "has got nothing to do with weapons of mass destruction. That is a different department altogether."

Whatever the team finds at Tuwaitha, it will probably be messy.

Dr. Hamed Al-Bahili, an Iraqi nuclear scientist who helped design and open Tuwaitha in 1968, was one of the first on the scene after fleeing Iraqi troops abandoned the site.

Raising his hand 2 inches above the linoleum floor in his living room, Al-Bahili said: "The uranium was all over the floor -- all over the ground outside. Piles of it. We poured cement over it inside the rooms because there was no other way to handle it."

Al-Bahili said he pleaded with impoverished villagers in the area not to touch the blue barrels the IAEA had used to store the uranium, "but there were thousands of people -- they just kept coming," he said in an interview Thursday at his Baghdad home.

Returning to Baghdad, he found Iraqi police who passed on his description of the scene and dangers to advancing U.S. troops.

Since then, Al-Bahili has met twice with U.S. military officials, whom he described as eager to help resolve the situation.

"They sent troops," he said, "but it was already too late."

U.S. troops involved in the hunt for weapons of mass destruction said recently that at least 20 percent of the barrels containing low-grade or natural uranium appeared to be gone.

Fleming said some 3,000 barrels were stored there under the agency's watch.

Last week, American troops accompanied by Iraqi health workers ordered residents from the surrounding villages to sell back barrels for $3 each. Pentagon officials said Thursday that more than 100 barrels had been retrieved.

Fleming said the IAEA would be permitted to examine its barrels. The rest of the mission, however, is restricted to the Tuwaitha site.

President Bush said he launched the war in part to disarm Iraq of weapons of mass destruction, although the U.N. teams found no evidence of the weapons Saddam's regime said it no longer possessed. U.S. weapons hunters, working from prewar intelligence assessments, also have failed to find any chemical, biological or nuclear weapons or unknown missile programs.

The lack of evidence has created pressure on both Bush and his coalition partners to explain the intelligence failings.

The British and Danish parliaments have launched investigations into the matter, and the CIA announced last week that it was reviewing its previous assessments.