A car bomb also hit members of a radical Shiite militia in northern Iraq as attacks nationwide killed at least 11 people Thursday.

In a videotape broadcast by the Al-Jazeera network, al-Qaida’s No. 2, Ayman al-Zawahri, threatened the United States with tens of thousands of military dead if it did not withdraw its troops from Iraq immediately.

Three U.S. soldiers were killed Wednesday night in a roadside bombing in Baghdad, the military said. The statement identified them only as members of the Army’s Task Force Baghdad, but Georgian officials said they were assigned to the 48th Brigade of the Georgia National Guard. The 48th has lost 11 soldiers since arriving in Iraq in May.

At least 1,825 members of the U.S. military have died since the Iraq war started in March 2003, according to an Associated Press count. Bombs in vehicles or planted alongside roads account for up to 80 percent of U.S. deaths in Iraq, military spokesman Lt. Col. Steven Boylan said.

The latest Marine casualty was in Ramadi, the capital of Anbar province 70 miles west of Baghdad. The Marine was killed by small-arms fire Wednesday &045; the same day 14 Marines and an Iraqi civilian translator died when a huge bomb destroyed their lightly armored vehicle near Haditha.

The death brought to at least 24 the number of Marines killed over the last week in the Euphrates Valley in one of the bloodiest periods for U.S. forces in months. In all, at least 48 American service members have died in Iraq since July 24 &045; all but two in combat.

Delaware Sen. Joseph Biden, the ranking Democrat on the Senate Foreign Relations Committee, called the Iraq campaign a tough situation ”that’s going to get tougher before it gets easier.”

”The truth of the matter is that we’ve made some pretty significant miscalculations in term of policy from the outset, and we leave these Marines in a very, very tough spot,” Biden said Thursday on CBS’s ”The Early Show.”

Seeking to reassure the public, Iraq’s prime minister announced a new plan for combating insurgents, declaring ”we are in a state of war.”

Prime Minister Ibrahim Al-Jaafari gave few details of the plan but said it was divided into 12 points and included steps to improve intelligence, protect infrastructure and prevent foreign fighters from infiltrating the country.

The U.S. command, meanwhile, said soldiers from the 3rd Armored Cavalry Regiment killed five insurgents who attacked Iraqi police Wednesday in the northern city of Tal Afar. There were no American casualties.

Elsewhere, gunmen attacked an Iraqi army patrol Thursday in Dujail, 50 miles north of Baghdad, killing four Iraqi troops, Brig. Ali Kadhim said.

Two Iraqi soldiers from the elite Wolf Brigade also were killed in a car bombing near a Shiite shrine in Daquq, 20 miles south of the oil-rich city of Kirkuk. The brigade members were accompanying a delegation from the Mahdi Army of radical Shiite cleric Muqtada al-Sadr back from Tal Afar, where they delivered supplies to beleaguered civilians.

The delegation had stopped at the shrine when the blast occurred. Two clerics from al-Sadr’s group also were killed, according to police Col. Mohammed Saleh Abbas. He said tensions in the area were running high after the blast.

In Baghdad, al-Sadr aide Sheik Abdul-Hadi al-Darraji blamed the U.S.-led coalition for failing to protect any areas except the Green Zone and warned ”if the government cannot confront terrorism, there are popular organizations that can,” referring to armed militias like the Mahdi Army.

Three other policemen were killed in a drive-by shooting in Kirkuk earlier Thursday, officials said.

In Baghdad, an aide to Iraqi Deputy Prime Minister Ahmad Chalabi was assassinated at his home early Thursday. Police said gunmen broke into the home of Haider Mohammed Ali al-Dujaili, head of the public relations department in Chalabi’s office, and shot him to death.

One of Chalabi’s security guards was killed in an ambush Sunday south of Baghdad. Chalabi, a former Pentagon favorite, was not in the convoy.

Violence is on the rise in Iraq despite plans by Bush to begin withdrawing American forces next year and gradually hand over security to the Iraqis.

In New York, the U.N. Security Council unanimously condemned recent terrorist attacks in Iraq and urged all countries to cooperate in bringing the perpetrators, organizers and sponsors to justice.

The bombing that killed the 14 Marines occurred near Haditha, 140 miles northwest of Baghdad, along a major infiltration route for foreign fighters entering the country from Syria.

A Marine officer, speaking on condition of anonymity for security reasons, said the troops were traveling in an armored amphibious vehicle to assault insurgent positions near the Haditha dam when a thunderous blast flipped the vehicle over and set it afire.

The Marines killed Wednesday were assigned to the 3rd Battalion, 25th Marines based in the Cleveland suburb of Brook Park, Ohio, and attached to the Regimental Combat Team-2. Nine of them were from a single smaller unit in Columbus, Ohio.

Bush called the attack a ”grim reminder” that America is still at war. U.S. leaders have been hoping that political progress toward a constitution and broadly elected government will dampen an intransigent insurgency.

As part of the political strategy, the Interior Ministry announced that a nationwide security operation was under way to protect voter registration centers for October’s constitutional referendum and December’s general election.

Minister Bayan Jabr said the monthlong operations began Aug. 1 but gave no details. As of Wednesday, 544 registration centers were opened throughout the country so 16 million voters could register, he said.