Rumsfeld appeared at a hearing of the Senate's Armed Services Committee where one of the least forgiving questioners was Democratic US Senator Hillary Clinton, a perennial critic, who lashed the defense secretary for a "record of incompetence."

"We hear a lot of happy talk and rosy scenarios, but because of the administration's strategic blunders and, frankly, the record of incompetence in executing, you are presiding over a failed policy," Clinton told Rumsfeld.

Rumsfeld answered that "history will make a judgment" about his alleged misjudgments and missteps.

"Are there setbacks? Yes. Are there things that people can't anticipate? Yes. Does the enemy have a brain and continue to make adjustments on the ground, requiring our forces to continue to make adjustments? You bet. Is that going to continue to be the case? I think so," he said.

"Is this problem going to get solved in the near term about this long struggle against violent extremism?" asked Rumsfeld.

"No, I don't believe it is, I think it's going to take some time."

Another Democrat, Carl Levin decried the rising sectarian violence, despite the heavy US military presence.

"Sectarian violence is not only on the rise, it has eclipsed the Sunni insurgency and the terrorism of Al-Qaeda in Iraq, in terms of the toll it has taken and the threats to Iraq's chances of stability," Levin said.

The beleaguered defense secretary had tried to avoid appearing before Thursday's panel pleading a busy schedule, but gave in to a chorus of calls by Senate Democrats who insisted it was vitally important he appear to explain US policy in Iraq.

Democratic Senator Ted Kennedy was also critical of Rumsfeld's leadership.

"We've been in there now for 40 months and 13 days with the finest military that's ever been developed in basically rather a third-rate military situation," Kennedy said.

Rumsfeld told Kennedy that ultimately the sectarian violence ragin across Iraq "is going to be dealt with by Iraqis, and it's going to be dealt with by Iraqi security forces as a part of the solution. But it's going to be dealt with through a reconciliation process, a political process."

Rumsfeld urged lawmakers not to withdraw from Iraq too early, saying it would only embolden extremists and lead to an eventual US withdrawal from the Middle East.

"We can persevere in Iraq or we can withdraw prematurely, until they force us to make a stand nearer home," the defense chief said.

"It is true that there are people who are attempting to prevent that government from being successful. And they are the people who are blowing up buildings and killing innocent men, women and children, and taking off the heads of people on television. And the idea of their prevailing is unacceptable."

His grilling at the hands of some Republicans on the committee was a bit milder, but still tough.

Senator John McCain, for one, was disheartened by developments in Iraq.

"It's very disturbing. And if it's all up to the Iraqi military ...then I wonder why we have to move troops into Baghdad to intervene in what is clearly sectarian violence," said McCain.

McCain, who served in Vietnam and was a prisoner of war, said a new plan to redeploy some US troops in Iraq is like a game of "whack-a-mole," where American forces try to quell sectarian violence in one area, only to have it surface elsewhere.

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