The nuclear experts will determine how much has gone missing from the nearly 2 tons of uranium and radioactive materials stored at the Tuwaitha plant outside Baghdad. U.S. commanders have said at least 20 percent of the uranium appears to be gone.
Their visit comes after U.S. military commanders acknowledged this week that after nearly three months on the ground, they remain unequipped to handle the nuclear site.
In other developments:
The arrival of the seven-member U.N. team — whose members are not weapons inspectors — marked the first time since the war began that representatives from the International Atomic Energy Agency, the U.N.'s nuclear agency, returned to the country.
The Pentagon has stressed the visit will not set a precedent for future U.N. searches for weapons of mass destruction in Iraq, which it has precluded.
With controversy swirling over the failure to find such weapons, chief U.N. weapons inspector Hans Blix said in an interview broadcast Friday that it is still too early to say whether Saddam Hussein had them before the war.
Blix came to a similar conclusion Thursday when he presented his final inspections report to the United Nations.
He told British Broadcasting Corp. radio that it would not surprise him if they were eventually found, but said he believed the U.S. and its allies had "other motivations" for wanting to invade.
Blix said the U.S.-led coalition had harbored real concerns about Saddam's possession of banned arms and the possibility terrorists could get hold of them.
"But judging by the discussion in the United States, there were a lot of other motivations why they wanted to start an armed action as well," Blix said.
Further clouding the issue was Friday's confirmation by Pentagon officials of a report last September by the U.S. Defense Intelligence Agency that it had no reliable evidence that Iraq had chemical agents in weaponized form.
But the agency also said Iraq probably had stockpiles of banned chemical warfare agents. The time frame coincides with Bush administration efforts to mount a public case for the urgency of disarming Iraq, by force if necessary.
A spokesman with the White House's National Security Council, Michael Anton, said a portion of the still-classified report was being taken out of context of the entire document's conclusions, which match what the Bush administration argued all along.
The lack of evidence so far has put pressure on British Prime Minister Tony Blair, who faces parliamentary scrutiny, and led to sharp domestic criticism for President Bush.