During a tense Senate Armed Services Committee hearing on Thursday, General John Abizaid, who as head of Central Command is the top US commander in the Middle East, declined to endorse Vice-President Dick Cheney's assessment that Iraq's uprising was in its "last throes".
Abizaid said the fighters' strength had not diminished and that more foreign combatants were coming into Iraq than six months ago.
"There's a lot of work to be done against the insurgency," the general said, adding: "I'm sure you'll forgive me from criticising the vice-president."
"This war has been consistently and grossly mismanaged," Senator Edward Kennedy, a Massachusetts Democrat, told Rumsfeld.
"And we are now in a seemingly intractable quagmire. Our troops are dying. And there really is no end in sight."
Kennedy added: "And the American people, I believe, deserve leadership worthy of the sacrifices that our fighting forces have made, and they deserve the real facts. And I regret to say that I don't believe that you have provided either."
"Well, that is quite a statement," Rumsfeld, flanked by top US commanders, responded.
"First let me say that there isn't a person at this table who agrees with you that we're in a quagmire and that there's no end in sight.
"The suggestion by you that people - me or others - are painting a rosy picture is false," Rumsfeld.
Several congressmen are asking
for a troop pullout timetable
"The fact is from the beginning of this we have recognised that this is a tough business, that it is difficult, that it is dangerous, and that it is not predictable," the defence chief added.
Kennedy asked Rumsfeld: "Isn't it time for you to resign?"
Rumsfeld noted he twice offered his resignation to President George Bush last year during the Abu Ghraib prisoner abuse scandal, and that Bush declined to accept it.
"That's his call," Rumsfeld added.
General George Casey, who commands the 138,000 US troops in Iraq, and General Richard Myers, chairman of the Joint Chiefs of Staff, agreed the war had not become a quagmire.
Sneer and disdain
Rumsfeld was chastised by the Senate's senior member, 87-year-old Democrat Robert Byrd of West Virginia, who objected to what he called the secretary's "sneer" and disdain towards lawmakers' questions.
"So get off your high horse when you come up here," Byrd told Rumsfeld, who later testified before a House of Representatives panel.
There have been 1725 US military deaths in a war that began in March 2003 and 13,074 US troops have been wounded, the Pentagon said.
"The suggestion ... that people - me or others - are painting a rosy picture is false"
US defence secretary
The May toll of 80 US troops was the highest since January and June's death count is on pace to match that of May.
There has been growing discomfort with the Iraq war among some US lawmakers, and support for the war by the American public has dropped in recent surveys.
Republican Senator Lindsey Graham of South Carolina said declining public support for the war was becoming a chronic problem
A new Iraqi constitution is due in August and elections for a new government are set for December.
"To the extent there were, for whatever reason, a delay in moving forward with drafting a constitution or a referendum on the constitution or holding the elections, it would retard the entire process," Rumsfeld said, but he did not identify possible consequences for a delay.
A small bipartisan group of lawmakers last week proposed a resolution calling on the Bush administration to develop a plan by the end of this year to pull out all American troops from Iraq and to begin the withdrawal by 1 October 2006.
Of a deadline, Rumsfeld testified: "It would throw a lifeline to terrorists, who in recent months have suffered significant losses in casualties, been denied havens, and suffered weakened popular support."
The committee's top Democrat, Senator Carl Levin of Michigan, said a deadline should not be ruled out.
Rumsfeld said Iraqi fighters have
suffered significant losses of late
"The Iraqis have approved a timetable for adopting a constitution: August 15th, with the possibility of one - and only one - six-month extension," Levin said.
"The United States needs to tell the Iraqis and the world that if that deadline is not met, we will review our position with all options open, including but not limited to setting a timetable for withdrawal," Levin said.
"We must demonstrate to the Iraqis that our willingness to bear the burden of providing security has limits. We have opened the door for the Iraqis at great cost, but only they can walk through it. We cannot hold that door open indefinitely," he added.