BAGHDAD — The Pentagon announced Tuesday that three more Americans serving in Iraq had been killed, lifting the U.S. military death toll to 2,000 and triggering a new round of protest and debate in the United States over the 2 1/2 -year conflict.

The new deaths were those of two Marines slain Friday in a roadside bombing near Amiriya, west of Baghdad, and a soldier who succumbed Saturday after being wounded last week by a similar blast in Samarra, north of the capital.

The fatalities reflected two troubling trends for the 140,000 U.S. troops stationed in Iraq — the growing loss of life caused by roadside bombs and the capacity of insurgents to re-infiltrate areas swept by U.S. offensives.

Thousands of Iraqis have been killed and more than 15,000 U.S. troops have been wounded since American-led forces invaded Iraq in March 2003 to topple President Saddam Hussein. At that time, Associated Press began a tally of U.S. military fatalities, and the casualties announced Tuesday brought the death toll to 2,000.

The milestone came amid declining support for the war in the United States, and activists planned a series of demonstrations.

Cindy Sheehan, the antiwar protester who lost a son in Iraq, began demonstrating Tuesday in front of the White House. MoveOn.org, a leading opponent of the war, planned candlelight vigils across the country in remembrance of troops who have died. The organization also planned to air TV ads about the passing of the 2,000 mark.

President Bush and top administration officials have made new appeals for patience in recent days, arguing that progress is being made.

"Iraq has made incredible political progress, from tyranny to liberation to national elections to the ratification of a national constitution in the space of 2 1/2 years," Bush said to applause Tuesday as he spoke to wives of military officers at Bolling Air Force Base in Washington, D.C.

While counseling that "no one should underestimate the difficulties ahead," Bush rejected the criticism that the war has prompted a new wave of terrorism. "I would remind them that we were not in Iraq on Sept. 11," he said.

Recently, some Republican lawmakers have begun to speak out in favor of reducing U.S. troop levels in Iraq, but the White House has been trying to damp such discussions. "Our strategy is to hold, clear and build," Secretary of State Condoleezza Rice said at a Senate hearing last week.

Tuesday's milestone drew sharply differing reactions from Congress. Democrats pointed to the losses and criticized the war, as Republicans complained that their opponents were trying to exploit the moment for political advantage.

Sen. Robert C. Byrd (D-W.Va.) called the moment "another tragic milestone in this costly war, in which too much blood has been spilled already."

But Sen. John Cornyn (R-Texas) said the figure was "an artificial landmark. Of course we grieve over each one of those losses, but it's an artificial number that some are using to try to undermine support for our effort there. These are people without any constructive alternative; is cutting and running what we're supposed to do?"

The Senate observed a moment of silence for the fallen troops. Democrats in the House read the names of all service members who had died.

As the number of deaths has climbed steadily and the insurgency shows no sign of weakening, even some Bush administration officials have begun to question a key assumption of American strategy: that the establishment of democracy in Iraq can erode support for and ultimately eradicate the guerrillas.

At the same time, senior U.S. commanders have acknowledged that the presence of American forces is inflaming the armed opposition.

While stepping up the training of Iraqi units to take over the fight, U.S. forces have continued to take the lead in striking the insurgency.

"The insurgents are still there," Army Maj. Gen. Rick Lynch, a U.S. military spokesman in Iraq, told reporters last week. "They still want to derail the democratic process. They still want to discredit the Iraqi government, so operations continue."

Of the three troops whose deaths were announced Tuesday, only one was identified by name. Staff Sgt. George T. Alexander Jr., 34, of Killeen, Texas, was wounded Oct. 17 in the town of Samarra, the Pentagon said, and died Saturday at Brooke Army Medical Center in San Antonio.