BAGHDAD, Iraq — An American citizen has been reported missing in Iraq, the U.S. Embassy said today, a day after a Canadian Parliament official said four humanitarian workers had been kidnapped.

Dan McTeague, parliamentary secretary for Canadians abroad, said two Canadians were taken on Saturday, and Britain said one of its citizens was among the four.

U.S. Embassy spokeswoman Elizabeth Colton said only that an American had been reported missing. The person's name was not released.

McTeague refused to name the organization the two Canadians worked for or the location where they were kidnapped in order to protect the safety of the individuals involved.

Briton Norman Kember was among the four, the British government said Sunday. His wife said he was representing a number of groups in the country and was a longtime peace activist.

Most international organizations fled Iraq last year following a wave of kidnappings and beheadings of foreign and Iraqi hostages. Many of them were carried out by al-Qaida in Iraq, led by the Jordanian Abu Musab al-Zarqawi.

Meanwhile, two Britons were killed and three injured today when gunmen attacked a bus carrying Muslim pilgrims south of Baghdad, police and hospital officials said.

The gunmen attacked the bus when it neared a checkpoint in the Dora neighborhood, police Capt. Talib Thamir said. The bus was carrying Shiite Muslim pilgrims to religious sites south of the capital, he said.

Four men and one woman, apparently of South Asian heritage and carrying United Kingdom passports, were taken to Baghdad's Yarmouk hospital, an official there said.

"We were just coming and all of a sudden heard shots and immediately got down," said an injured British woman, who identified herself as Z. Jafferti. "I don't know what happened and I couldn't see anything."

She said she had been ill and had come to Iraq to pray at the shrines to Imam Ali and Imam Hussein.

Also this morning, a mortar shell fell in central Baghdad's Green Zone and two others fell nearby, just hours before Saddam Hussein's trial was set to begin. There were no report of injuries from the shelling, police Lt. Bilal Ali Majeed said.

A roadside bomb also detonated next to a passing U.S. Army convoy in northeastern Baghdad today, setting fire to a Bradley fighting vehicle. Police Capt. Mohammed Abdul-Ghani said three soldiers were injured, but no other information was available.

The U.S. military reported that a Marine assigned to the 2nd Marine Aircraft Wing was killed Saturday when his vehicle struck a roadside bomb near Camp Taqaddum, 45 miles west of Baghdad.

Near Kirkuk, 180 miles north of Baghdad, soldiers from the 101st Airborne Division found more than 2,700 mortar rounds buried near an abandoned Iraqi Army base, a U.S. statement said. Troops were excavating similar mounds today in search of more weapons.