The U.S. military said Thursday that another Marine was killed in action in the same Euphrates River (search) valley where 14 Marines died in the worst roadside bombing targeting American forces in the Iraq war.

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

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

The latest death brought to at least 24 the number of Marines killed over the last week along the Euphrates Valley in one of the bloodiest periods for U.S. forces in months. In all, at least 45 American service members have died in Iraq since July 24 — 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."

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.

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 Monday. 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 President Bush to begin withdrawing American forces next year and gradually hand over security to the Iraqis.

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.

At least 1,822 members of the U.S. military have died since the Iraq war started in March 2003, according to an Associated Press count.

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.