United Nations nuclear experts have begun to investigate the looting of material from Iraq's main nuclear facility after the war.
The US has severely limited the scope of the team's mission
The team of seven from the International Atomic Energy Agency (IAEA) will spend two weeks at the Tuwaitha complex, 50 kilometres (30 miles) south of Baghdad, determining what is missing, dealing with what is left and making it safe.
As the visit got under way the row over the main US justification for the war in Iraq - the alleged presence of weapons of mass destruction (WMD) - intensified.
A classified September 2002 Pentagon intelligence report leaked to the US media concluded that there was "no reliable information" that Iraq had biological or chemical weapons.
WERE WE MISLED OVER WMD?
You don't think that Saddam was knitting for the last few years after he kicked out the inspectors? Have some patience.
The report contradicted Defence Secretary Donald Rumsfeld's claim at the time that Iraq had amassed large stockpiles of nerve agent and mustard gas.
The CIA has launched an investigation to see if intelligence reports were doctored to exaggerate the threat posed by Saddam Hussein.
US forces have not yet found any WMD in Iraq - and two suspect mobile labs found there have not provided any such evidence.
The UN chief weapons inspector, Hans Blix, has criticised the quality of intelligence given to him by the US and Britain about Iraq's alleged WMD.
Mr Blix told the BBC that his teams followed up US and British leads at suspected sites across Iraq, but found nothing when they got there.
The UN experts at Tuwaitha are being blocked from investigating the reports of contamination and sickness by the United States, which argues that as the occupying power it is responsible for the health of the Iraqi people.
The number of inspectors has been limited by the Pentagon to seven, and their assessment has to be completed in two weeks.
Local people have been using barrels that held highly radioactive material to store food or wash clothes and some have complained of subsequent health problems, such as nose bleeds and vomiting.
The inspectors were allowed in after the IAEA director, Mohamed ElBaradei, said a radiological emergency could be brewing at the plant after looters left behind piles of uranium and spilled radioactive materials.
Local people say looters were not after the uranium itself. They tipped it onto the ground so they could take away the containers to store food and water, the BBC's Caroline Hawley reports from Baghdad.
Workers living on the site, worried about being contaminated themselves, buried the spilled uranium in cement and made a desperate appeal to visiting journalists for international help, our correspondent adds.
Before the war, the site held two tons of low-grade enriched uranium and several tons of natural uranium. A storage facility near the site held several hundred other radiological sources.
The IAEA will check stocks of enriched uranium and "yellow cake", or processed mined uranium, against its detailed inventory lists.
US defence officials quoted by the Reuters news agency insist that US troops accompany the UN inspectors at the site, and that the visit sets no precedent for a future IAEA role in Iraq.
The IAEA team are nuclear experts but not weapons inspectors. The Americans will deal with the disarmament of Iraq themselves - a US-backed team of 1,400 inspectors is due in the country in coming days.
They are expected to fan out across the country to continue the search for WMD.
On Thursday, Mr Blix gave what is likely to be his last report to the UN Security Council before he retires later this month.
Mr Blix's briefing recorded an open verdict over whether Iraq had WMD.
He told the BBC he had been disappointed with the tip-offs provided by British and US intelligence.
"Only in three of those cases did we find anything at all, and in none of these cases were there any weapons of mass destruction, and that shook me a bit, I must say."
"I thought - my God, if this is the best intelligence they have and we find nothing, what about the rest?"