United Nations nuclear inspectors have left their Vienna headquarters for Iraq, for the first time since the US-led invasion.
Troops came in after reports that the facility had been looted
The seven experts from the International Atomic Energy Agency (IAEA) will check reports of widespread looting at Iraq's largest nuclear site. They left for Kuwait on Wednesday, and are not expected to arrive in Baghdad before Friday.
Looters reportedly removed nuclear containers from the sprawling Tuwaitha nuclear facility, raising fears of radioactive contamination.
However, the IAEA is not being allowed by the US-led occupying powers to resume its hunt for banned weapons of mass destruction which was suspended on the eve of war.
We don't want nuclear material anywhere lying around. We want to make sure it does not fall into the wrong hands
IAEA spokeswoman Melissa Fleming
The United States only agreed to let the IAEA mission into Iraq late last month, after repeated calls by the IAEA head, Mohammed ElBaradei.
Meanwhile, chief UN weapons inspector Hans Blix said in his latest report on Tuesday that his inspectors had found no evidence to show that Iraq had been running programmes to build weapons of mass destruction.
The IAEA team will only have access to a small area outside the Tuwaitha site, which is about 50 kilometres (30 miles) south of Baghdad.
ElBaradei warned that Iraq could face nuclear contamination
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.
"We don't want nuclear material anywhere lying around. We want to make sure it does not fall into the wrong hands," IAEA spokeswoman Melissa Fleming told reporters at Vienna airport.
But it has only been authorised to check whether nuclear material under IAEA seal since 1991 remains safe and accounted for and that Iraq is abiding by its obligations under the Nuclear Non-proliferation Treaty.
The inspectors are not authorised to look at nuclear health and safety issues.
The IAEA wrote a letter to Washington in early May requesting access to nuclear sites in Iraq after reports of looting initially surfaced.
It had previously urgently recommended that the Tuwaitha site and facilities at Baghdad be given protection.
Mr ElBaradei expressed frustration that more attention had not been paid to securing such sites, warning that Iraq could be facing a potential nuclear contamination emergency.
It was a further souring of relations already strained by the ejection of UN inspectors from Iraq before they had declared their mission over.
It appeared that US soldiers had been sent to some sites but were unwilling or unable to seal them off.