The Washington Post on Tuesday reported that the US Central Intelligence Agency (CIA) and other intelligence agencies warned that there would be significant hostility to a US-led occupation.
The post-war period would pose more problems than the war itself, the daily quoted the agencies as saying.
The post-war scenarios presented by the CIA and its counterpart agencies in the Pentagon and State Department were more pessimistic than senior Pentagon officials.
"Intelligence reports told them at some length about possibilities for unpleasantness," said a senior administration official, who like others spoke on condition of anonymity.
"The reports were written, but we don't know if they were read," he added.
The post-war threats outlined by the intelligence community included the possibility that the "Iraqis probably would resort to obstruction, resistance and armed opposition if they perceived attempts to keep them dependent on the US and the West," a senior congressional aide said.
The CIA believed that members of deposed Iraqi President Saddam Hussein's elite Republican Guard and his Baath party had plans to carry on resistance efforts after the war, said a senior intelligence official.
"The reports were written, but we don't know if they were read"
Senior US administration official
They had been given instructions should the government fall, the official added.
Meanwhile, thousands of United States’ National Guard and Army Reserve troops currently occupying Iraq are to have their tour of duty extended to a year, months longer than anticipated.
The order to extend the original year-long tour for one to six additional months was issued late on Friday, but no formal announcement has been made, the Washington Post said on Tuesday, quoting US Army officials.
The new policy, which experts say is likely to have a negative impact on morale, retention and recruiting, coincides with a drive by the US government to recruit more international help in Iraq through the United Nations.
Of the 122,000 US troops currently deployed in Iraq, according to the Post, 3000 belong to the National Guard and 5000 are reservists. Another 5000 guards and 7000 reservists are serving in Kuwait.
Army officials defended the new policy, saying that the scarcity of active-duty forces and security concerns made it necessary to keep a large number of Guard and Reserve troops in Iraq for as long as possible.
The US has faced continuous opposition from Iraqi resistance fighters since President George W Bush declared combat operations over on 1 May. Since then, close to 70 US soldiers in Iraq have died in guerrilla attacks.