WASHINGTON (AP) — Defense Secretary Donald Rumsfeld steadfastly declined Thursday to give Congress a public estimate of the size of the Iraqi insurgency.

Under persistent questioning by Sen. John McCain, R-Ariz., Rumsfeld said the disorganized nature of the insurgency make it difficult to pin down a reliable, specific estimate.

"They're not static. The numbers change," he told the Senate Armed Services Committee. "They're made up of different elements: criminals, Baathists, the former regime elements, the Zarqawi network and jihadists. Even though the jihadists are the smallest portion of them, they appear to us to be the most lethal."

Still, McCain pressed for numbers.

"Shouldn't the American people also know the size and shape and nature of the enemy that we're facing, since it's their sons and daughters who are going to serve?" he asked. Rumsfeld said it was not his place to declassify the estimates provided to him by intelligence services.

The defense secretary had refused Wednesday to give such an estimate to a House committee.

Gen. Richard Myers, chairman of the Joint Chiefs of Staff, told the Senate panel that although the numbers are uncertain, the military believes they have gauged their capabilities.

"They have a limited capacity," he said, adding that they can conduct 50 to 60 attacks per day around the country.

On Wednesday, before the House Armed Services Committee, Rumsfeld was quoted an estimate by an Iraqi official that there were as many as 40,000 insurgents and 200,000 part-time supporters in Iraq. He responded that those figures were much higher than estimates of the Central Intelligence Agency and Defense Intelligence Agency, which also differ from one another.

"Frankly, I don't have a lot of confidence in any of them (estimates)," Rumsfeld said, declining to provide the CIA and DIA numbers.

Military officials estimate 15,000 insurgents have been killed or captured in Iraq.

However, those who leave Iraq will carry their experience elsewhere, CIA Director Porter Goss said Wednesday in congressional testimony.

"Those jihadists who survive will leave Iraq experienced in and focused on acts of urban terrorism. They represent a potential pool of contacts to build transnational terrorist cells, groups and networks," Goss said.

Also in testimony Wednesday and Thursday, Rumsfeld acknowledged that American forces were in Iraq conducting clandestine operations before the U.S. invasion began in March 2003.

"While the negotiations were still going on in the United Nations with respect to Iraq, the Department of Defense engaged in some entry into Iraq with clandestine operatives to prepare the battlefield, to learn the things they needed to know in the event something were to take place," Rumsfeld told a House subcommittee Thursday.

He did not provide specifics.

A military official, speaking on condition of anonymity, said another operation involved special operations forces protecting Iraqi oil fields by preventing Saddam's forces from blowing them up as soon as the invasion began.