US forces have found no banned weapons so far
A lorry found in northern Iraq was probably used as a mobile factory to produce biological weapons, the Pentagon has said.
The announcement comes after the US said on Tuesday it was conducting tests on the lorry.
Pentagon intelligence chief Steve Cambone said the lorry - which matches the description of a mobile biological weapons lab given by various sources including a defector before the war - was found and seized by US forces at the end of April near the northern town of Mosul.
"US and UK technical experts have concluded that the unit does not appear to perform any function beyond what the defector said it was for, which was the production of biological agents," he said.
This is the first time a top Pentagon official has said the US military has in its possession what looks like evidence of an Iraqi weapons of mass destruction programme, says BBC Pentagon correspondent Nick Childs.
The US cited concerns about alleged weapons of mass destruction as its primary reason for invading Iraq in March, but so far none have been found.
Dr Cambone said the lorry, painted in a military colour scheme, came into the hands of US forces on 19 April at a Kurdish checkpoint near the town of Tall Kayf in
Officials such as Tariq Aziz have apparently revealed nothing
It was found on a heavy-equipment
transporter typically used for carrying tanks, he added.
The vehicle is the kind of mobile laboratory described to the United Nations' Security Council by US Secretary of State Colin Powell before the conflict began, Dr Cambone said.
Aboard the lorry was equipment that can be used to make biological weapons - living micro-organisms used deliberately
to spread disease - including a fermenter that could help produce the germ warfare agents, he said.
But the Pentagon remains somewhat cautious and Dr Cambone would not call this a smoking gun, our correspondent says.
Dr Cambone said more tests will be required to see if there is evidence of actual biological agents.
He said the lorry's surface had been washed with a caustic material
and that it would probably have to be taken apart for extensive testing.
Officials are being cautious after several previous discoveries turned out to be false alarms.
Preliminary tests of suspected weapons facilities sometimes turn up false positives because many of the same substances are used in both weapons and fertiliser.
But US President George W Bush said he was confident that banned weapons would be found in Iraq.
He told reporters on Tuesday he would not be surprised if the lorry turned out to be part of a mobile laboratory because Saddam Hussein "had a weapons programme".
At the weekend he said it was only a "matter of time" before US troops find banned weapons in Iraq.
"Saddam Hussein had weapons of mass destruction," Mr Bush said at his ranch in Crawford, Texas. "It's well known."