During a hearing of the House of Representatives Armed Services Committee on Thursday, Rumsfeld declined to publicly answer lawmakers who asked him the numerical strength of the anti-US forces fighting the roughly 150,000 US troops in Iraq.
"The intelligence community looks at that. The CIA does, DIA (Defence Intelligence Agency) does, others do. And they have differing assessments," Rumsfeld said.
"My job in the government is not to be the principal intelligence officer and try to rationalise differences between Iraqis, the CIA and the DIA. I see these reports. Frankly, I don't have a lot of confidence in any of them, on that number," Rumsfeld said.
In a Senate testimony later, Rumsfeld also said the Pentagon had no plans for a permanent US troop presence in Iraq, but did not rule out such an arrangement with a future elected government.
He also suggested the US may be able to bring home its troops within 18 months if the anti-US fighting was contained.
Fighters have waged a bloody
campaign against US forces
The Pentagon has struggled to gauge the size, composition and organisation of the fighters who have waged a bloody campaign to dislodge all foreign forces since a US-led invasion in 2003.
Rumsfeld said he would not give a number because it was not his business to do intelligence work. He added that he could not reveal CIA and DIA estimates because they were classified.
Rumsfeld did not explain his lack of confidence in the estimates. But he has been outspoken about the need for more and better human intelligence to be gathered by the US intelligence community.
General Richard Myers, chairman of the US Joint Chiefs of Staff, disputed an estimate of the size of the anti-US forces offered recently by General Muhammad Abd Allah Shahwani, director of the Iraqi intelligence service.
Myers disputed Iraqi estimates
of anti-US forces' strength
Shahwani had said there were 200,000 anti-US fighters, including at least 40,000 hardcore fighters, with the remainder being part-time fighters and supporters who provide money, intelligence, food and shelter.
Myers said US estimates were considerably lower. Rumsfeld called Shahwani's numbers "totally inconsistent" with US estimates.
Critics have accused Rumsfeld of encroaching on the CIA by expanding Pentagon intelligence operations.
Rumsfeld said the Pentagon expected 200,000 Iraqi security personnel to be trained and equipped by late 2005 before elections on a new constitution. That would be an increase from the current estimated 136,000 Iraqi security personnel.
He said he expected 230,000 to be in place by December 2005 or January 2006 for the next round of elections, with the ultimate goal of 270,000 by June 2006.
"My job in the government is not to be the principal intelligence officer and try to rationalise differences between Iraqis, the CIA and the DIA"
US defence secretary
But the timetable for withdrawing US troops also became an issue at the Senate Appropriations Committee hearing.
New Mexico Republican Senator Pete Domenici asked Rumsfeld: "If the end of this war is when they [Iraqis] no longer need us, how close are we to having them trained so that we will not be needed anymore? Could they be ready in two years, in a year in a half?"
Rumsfeld responded: "It could be before that," depending on when the anti-US campaign can be subdued.