Ahmed Chalabi, a key member of the US-picked interim government, said information received on August 14 was passed on to the US authorities.
"We received information that a large-scale terror attack would take place in Baghdad," he said. "The information said the attack would be aimed at a soft target, not the American military or forces.
"The information said the attack would use a truck and would be carried out by using a suicide mechanism or by remote control. We shared this information with the Americans," Mr Chalabi said.
At least 20 people were killed in the bombing, including the UN special envoy to Iraq, Sergio Vieira de Mello, and the death toll was expected to rise as the rubble was cleared.
The FBI said the bomb was made from 1,000lbs of old munitions including one single 500lb bomb, all of the materials from Saddam Hussein's pre-war arsenal that required no "great degree of sophistication" to build.
One agent at the blast site said it was impossible to say yet whether the attack was the work of Saddam loyalists or foreign terrorists. Mr Chalabi insisted the bomb was the work of Saddam loyalists, but he gave no evidence.
FBI Special Agent Thomas Fuentes said the bomb had been delivered by a KAMAZ flatbed truck.
"We believe the bomb was made from existing military ordnance. I cannot say that it required any great degree of sophistication or expertise to create" Mr Fuentes said.
Human remains were found in the area where the bomb exploded, he said, suggesting a suicide bombing. He said that could not be absolutely determined until lab tests were complete.
In Stockholm, UN Secretary-General Kofi Annan said the unprecedented blast would not drive the world body out of Iraq.
"We will persevere. We will continue," he said. "It is essential work. We will not be intimidated".
"We have been in Iraq for 12 years and we have never been attacked," Mr Annan said. He said the UN would now re-evaluate its security measures.
Unlike the US occupation forces, the UN had been welcomed by many Iraqis and there was no clear indication of who was behind the attack on the three-storey building. No group has claimed responsibility.
"There are at least two hypotheses," said the top US official in Iraq, Paul Bremer. One blamed remnants of the Saddam regime, he said, and the other insurgents from overseas.
The bombing came nearly two weeks after a car exploded and killed 19 people at the Jordanian Embassy in Baghdad and after a string of dramatic attacks on oil and water pipelines in Iraq.
Tuesday's attack resembled others blamed on Islamic militants elsewhere in the world.
While the bomb used was not sophisticated, the operation as a whole showed greater planning and more precise targeting that the guerrilla attacks that have plagued US forces.