WASHINGTON (AFP) - General David Petraeus said Monday the US surge in Iraq was working and troop numbers could begin to ebb in December, but warned Congress that a large-scale withdrawal would be "devastating."

The Iraq war commander battled repeated interruptions by anti-war protestors in a landmark hearing, and rejected angry Democratic claims that President George W. Bush's war plan was a failure.

"The military objectives of the surge are in large measure being met," Petraeus, in green dress uniform bearing a chestful of medal ribbons, told lawmakers.

The rare joint hearing of the House Armed Services and Foreign Relations committees opened a pivotal week for US Iraq policy in a four-year war which has killed more than 3,700 American troops and tens of thousands of civilians.

Democrats, who have consistently failed in their attempts to force Bush to bring most combat troops home by early next year, argued that the surge of 28,500 US troops had failed in its goal of driving political reconciliation in Iraq.

But using graphs and colored charts, the general, accompanied by US ambassador to Baghdad Ryan Crocker, warned "a premature drawdown of our forces would likely have devastating consequences."

But he said gradual troop withdrawals were feasible and would reduce troops near to pre-surge levels by the middle of next year.

A marine expeditionary unit of about 2,000 troops would leave Iraq later this month and not be replaced, and an army combat brigade of about 4,000 troops will redeploy in December.

The impact of reductions would be to effectively end the surge in 10 months.

Before the surge started in February, there were around 130,000 US troops in Iraq.

Military officials had previously acknowledged that continuing the surge after mid-2008 would be difficult because of the lack of available forces to keep the level of troops so high.

But angering Democrats, Petraeus said further decisions on troop numbers could not be made until next March at the earliest.

"In my professional judgment, it would be premature to make recommendations on the pace of such reductions at this time," Petraeus said.

He said civilian deaths had "declined significantly" under the surge. A US official auditor last week however questioned the way the military had calculated the statistics on sectarian violence.

Petraeus also warned that Iran was using its special forces to fight a "proxy war against the Iraqi state and coalition forces in Iraq."

Crocker added that "Iran would be a winner" if the US abandoned Iraq, and argued a "secure, stable, democratic Iraq at peace with its neighbors is, in my view, attainable."

The general hailed the decision by Sunni tribes in Anbar province to join the fight against Al-Qaeda as "the most significant development in the past six months."

Police swarmed the hearing, evicting nine anti-war demonstrators -- including renowned protestor Cindy Sheehan.

One woman screamed "No, No, No, No," as she was dragged out.

As political tensions hit boiling point, Foreign Affairs committee chairman Tom Lantos told Petraeus he didn't "buy" claims that victory was at hand.

"The current escalation in our military presence in Iraq may have produced some tactical successes. But strategically, the escalation has failed," he said accusing Iraqi leaders of squandering an opportunity for political reform.

"We need to get out of Iraq, for that country's sake and for our own. It is time to go."

Democratic Congressman Robert Wexler asked "how many more men and women will (be) sacrificed to protect our so called credibility?"

Petraeus replied: "no one is more concious of the loss of life than the commander of the forces, that is something I take and feel very deeply."

Political fury over the war spilled over even before Petraeus and Crocker appeared, with Republicans accusing Democrats of embracing "character assassination" tactics designed to discredit the general's testimony.

The flare-up was sparked by a full-page advertisement in The New York Times by liberal group MoveOn.org, reading "General Petraeus or General Betray US? Cooking the Books for the White House."

The general and ambassador were also due to appear before the Senate Foreign Relations and Armed Services committees on Tuesday.

Bush, bound by law to provide a report on the progress of the war by Saturday, was expected to make a televised address to the American people later in the week.