After fresh denials from the regime it has any prohibited arms or banned military programmes, at least three UN teams were at work in the field testing the claims.
Inspectors from the UN Monitoring, Verification and Inspection Commission (UNMOVIC) went back to the giant Al-Fao engineering plant in the Al-Kremiat area, which they had already visited on Saturday.
A biological team entered the baby milk factory at Abu Ghreib, 20 kilometres (12 kilometres) northwest of the capital.
Suspected of secret biological weapons production, the factory was totally destroyed during the 1991 Gulf war and bombed again in 1998 after reconstruction.
A third team specialised in chemical weapons was working at the Ibn al-Bitar centre for veterinary medicine research, 18 kilometres (11 miles) north of Baghdad. It is run by the industry ministry's Al-Razi company.
Iraq renewed Sunday cautious praise for the efforts of the inspectors to remain neutral, but the official press lashed out Monday at the United States.
President George W. Bush and British Prime Minister Tony Blair were waging a "rabid campaign" against Baghdad, the ruling Baath Party charged.
"Every time the Iraqi question needs to be dealt with calmly, the administration of little Bush and his valet Blair launch a rabid campaign of blatant lies and false accusations to prime the US and Zionist media," said the party daily, Ath-Thawra.
The US administration "is lying, throwing out accusations at will, raising doubts and making assumptions.
"When it runs into a dead-end and has to turn its allegations into solid proof, it invents hollow excuses," Ath-Thawra said.
The government newspaper Al-Jumhuriya stressed that accusations Iraq possesses weapons of mass destruction "are just baseless lies intended to support an attack on Iraq to take over its oil wealth."
UN inspectors had probed six sites on Sunday, including a space research centre, as the United States started sharing sensitive intelligence with the UN inspectors.
The same day a top advisor to Iraqi President Saddam Hussein said the Central Intelligence Agency was welcome to come and check for itself the sites it has targeted.
"We have no problem with the American intelligence services coming to Iraq and pointing out to the inspectors the sites that they should inspect," said General Amer al-Saadi, Saddam's top adviser on weapons.
"After 24 days of inspections covering practically all the sites named in those reports and after the submission of our declaration of December 7, the lies and baseless allegations have been uncovered," Saadi said.
Saadi said Iraq had no more documentation to support its claims "but we are ready ... to work and cooperate with UNMOVIC to find ways of resolving the remaining disarmament issues, provided also that an assessment is made of the significance of those remaining issues."
He also paid tribute to the UN inspection agency experts as "fully objective".
UNMOVIC, which resumed groundwork on November 27 after a four-year break from UN inspections, is tasked with disarming Iraq under UN Resolution 1441, which threatens serious consequences if Baghdad fails to cooperate fully.