"I am not going to give you a number for it because it's not my business to do intelligence work," Rumsfeld said.

Testifying before the House Armed Services Committee, Rumsfeld sketched out plans to nearly double the size of the Iraqi security forces by mid-2006, building it up from 136,000 currently to 270,000.

The plans call for having 200,000 trained security forces in time for a referendum on an Iraqi constitution in September or October, and 230,000 by the end of the year for the next elections for a permanent government, he said.

But he would not predict how long it will take before the Iraqi security forces are capable of defeating the insurgency on their own.

"You say, 'how long will it take?' The goal is to have the Iraqis have the security capability to manage their own insurgency," Rumsfeld said.

"The question is: how does that capability match with the actual problem of the insurgency? Is the insurgency going to grow or decline?"

The size and make-up of the Iraqi insurgency has long been a significant unanswered question with broad implications for the US strategy in Iraq.

Representative Ike Skelton, the committee's ranking Democrat, noted US military estimates that 15,000 insurgents have been killed or captured over the last seven months.

"If we'd lost 15,000 of our own troops in a comparable period, we would see diminished combat capability. And yet our enemy seems to be adapting and increasing his attacks," said Skelton, who criticized Rumsfeld for lacking "a strategy for success" nearly two years after the US-led invasion of Iraq.

The congressman asked the secretary about a recent estimate by Iraq's intelligence director that the insurgency has grown to some 40,000 hardcore fighters with 200,000 part-time fighters.

Rumsfeld responded that the Central Intelligence Agency and the Defense Intelligence Agency have had differing assessments at different times of the insurgent strength.

He took out a piece of paper that he said contained the intelligence agency estimates, but when asked to share them in the open session, said they were classified.

"My job in the government is not to be the principal intelligence officer and try to rationalize 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," he said.

General Richard Myers, the chairman of the Joint Chiefs of Staff, said the insurgents were a conglomeration of former regime loyalists, foreign jihadists, unemployed Iraqis and criminals.

"So the number is, like all insurgencies, very hard to measure," he said.

"These are generally small cells. They don't keep very good records. As we capture individuals you can't simply go into their paraphernalia and say, 'Ah-ha, here's their organizational diagram,' because they don't have one. So the estimates are hard to come by," he said.

But he said the insurgents had not used conventional military tactics outside of Fallujah, the resistance stronghold that was retaken in November in a bloody assault led by marines.

"They use terrorism. They use extremism. They use beheadings. And that's probably the level of their capability, despite what the numbers might be," he said.

He said knowing their numbers wasn't as important to defeating them as good government, good economic conditions and infrastructure and effective public communication policies.

They were testifying about the Pentagon's proposed 419 billion-dollar 2006 defense budget, but most questions revolved around Iraq and an 81.9 billion-dollar request for supplemental funding in 2005.

Rumsfeld said there will be a "modest refocusing" of the US military effort on training and advising the Iraqi security forces.

Myers and Rumsfeld both acknowledged the "significant" stress placed on US forces. Myers said he was concerned in particular about the accelerating wear and tear on equipment.

Noting the heavy reliance on national guard and reserve forces, he predicted "a very challenging year" for recruiting members of the guard and reserve.

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