"Iraq has concealed several years of biological warfare programmes, consisting of a rare weaponised system that can be put into bombs and missiles," UN disarmament chief, Rolf Ekeus, said in Amman.
And he said that even before the start of the 1991 Gulf war, Baghdad had "prepared biological warheads".
Baghdad had also produced vast quantities of deadly botulism and anthrax agents, far greater than it had admitted to in the past, Mr Ekeus said. "Iraq has produced more anthrax than stated," he added.
Baghdad said the agents had been destroyed, but Mr Ekeus warned the information handed over during a trip to Iraq last week would have to be verified.
UN experts who arrived on Tuesday in Baghdad would be checking everything.
Iraq has systematically concealed the true size and nature of its weapons stocks since it was defeated by a US-led coalition in the January-February 1991 conflict which ended its seven-month occupation of Kuwait.
Since 1993 Iraq has regularly maintained that it has fully complied with UN Security Council resolutions demanding it destroy its weapons of mass destruction.
But its protests were brushed aside by the UN Special Commission in charge of disarming Iraq, Unscom, which tried to force Iraq to tell the truth and was helped by information from Baghdad's former suppliers.
The defection to Jordan on 8 August of General Hussein Kamel Hassan, considered the architect of the arms programmes, forced Baghdad to hand over information before the defector did.