Rumsfeld also vehemently denied that intelligence analysts were pressured to tailor conclusions to fit the administration's call for military force.

In a tense hearing, Democratic senators on the Senate Armed Services Committee confronted Rumsfeld with his own bald pre-war statements that Iraq had chemical and biological weapons and the United States knew where they were.

Rumsfeld acknowledged that he may have once overstated in saying he knew where Iraq's weapons were located, but he maintained that the administration acted on a consistent stream of intelligence developed since the administration of President Bill Clinton.

"Senator Warner asked in his opening statement if I know of any pressure on intelligence people or manipulation of intelligence, and the answer is absolutely not," Rumsfeld said, referring to the committee's Republican chairman, John Warner.

The hearing was called to review the Pentagon's request for a 401.7 billion dollar budget, but it was the Senate's first opportunity to question a top member of the administration about the failure to find chemical, biological or nuclear arms.

Senator Ted Kennedy, a leading Democrat, called it "a devastating refutation of the Bush administration's case for war in Iraq and I think seriously undermines our credibility in world."

David Kay, leader of the US weapons search, told Congress last week he had concluded there were no banned weapons in Iraq, and no evidence that they existed before the war.

"I suppose that's possible but not likely," Rumsfeld said of Kay's view.

He said the Iraq Survey Group that Kay has quit would keep working and it was too soon to draw conclusions about what happened to the weapons that the intelligence community believed were there.

"As Dr Kay has testified, what we have learned thus far has not proven Saddam Hussein had what intelligence indicated and what we believed he had. And it also has not proven the opposite," Rumsfeld said.

He highlighted theories that weapons were transferred to other countries, that they were hidden throughout Iraq, that they were destroyed before the invasion, or that there were small stockpiles prepared for a rapid buildup.

Rumsfeld said the size of Iraq would make it easy for things to be hidden, recalling that it took 10 months to find ousted president Saddam Hussein.

"The reality is the hole he was hiding in was large enough to hide enough biological weapons to kill thousands of human beings," he said.

Senator Carl Levin, the ranking Democrat, contrasted a September 2002 report by the Defense Intelligence Agency that said there was "no reliable information" on Iraq's producing chemical weapons with a Rumsfeld assertion the same month that "we know" Iraq was hiding biological and chemical weapons in residential neighborhoods.

"I have relied not on any one single intelligence entity, like the DIA or the CIA. I've relied on the intelligence community's assessments. And the intelligence community's assessments were what they were. And they were as I stated them," Rumsfeld said.

Levin then asked about a special Pentagon unit that critics have charged was used to "cherry pick" intelligence that fit the administration's case for war.

"Their task was to take the intelligence that existed and look at it and see what they could figure out about it, just as I do when I read it and you do when you read it," Rumsfeld replied.

Kennedy charged that the White House's agenda was to blame the failure of its case for war on the intelligence community, rather than its own "manipulations and misrepresentations on the available intelligence."

"Key policy-makers made crystal clear the results they wanted from the intelligence community," he said.

Rumsfeld shot back: "You've twice or thrice mentioned manipulation. I haven't heard of it. I haven't seen any of it except in the comment you have made," he said.

Levin challenged that with a comparison of intelligence estimates that showed assessments were hardened significantly in October 2002 to find Iraq had reconstituted its nuclear program, enriched uranium for nuclear weapons and had large stockpiles of chemical weapons.

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