The Marez base is shared by U.S. troops and Iraqi security forces who together have fought a stubborn insurgency in the city of 1.8 million, after its police force dissolved in the face of a massive insurgent offensive last month.
Security at all U.S. bases is ordinarily extremely tight, and any penetration by a suicide bomber would be a startling breach. Local Iraqi workers typically are searched before entering and monitored while on base. Iraqi soldiers are the only local nationals usually seen in mess halls.
Meanwhile, two French journalists were released after being held hostage for four months by Muslim extremists based near Baghdad. Christian Chesnot of Radio France International and Georges Malbrunot of the newspaper Le Figaro flew to Jordan accompanied by French authorities, news agencies reported.
Also Tuesday, British Prime Minister Tony Blair paid a surprise visit to Baghdad. Britain has 8,000 troops in Iraq, the most of any country after the United States, which has about 150,000.
At a news conference in the Green Zone, Blair spoke of the sense of danger evident even within the heavily fortified section of the capital reserved for Iraq's interim government and the American and British delegations that support it.
Blair singled out as "heroes" the country's 6,000 election workers, three of whom were executed in broad daylight on a Baghdad street two days earlier.
Blair called the Jan. 30 elections a contest between "democracy and terrorism" and praised the election workers for "risking their lives every day in order to make sure that the people of Iraq get a chance to decide their own destiny."
He then flew to the southern city of Basra, where British forces have been based since the March 2003 invasion except for several weeks this autumn when a battalion conducted operations and took significant casualties against an insurgent stronghold south of Baghdad. Seventy-four British troops have been killed in Iraq.
Blair vowed to maintain forces in Iraq, saying, "If Iraq is a stable and democratic country, that is good for the Middle East, and what is good for the Middle East is actually good for the world, including Britain."
But Italian news agencies reported that Italian Prime Minister Silvio Berlusconi said his country would begin discussing a phased withdrawal of its 3,000 troops from Iraq after the Jan. 30 election. Berlusconi said he and Blair discussed trying to persuade Iraq's new government to make firm commitments for establishing its own security forces in order to allow the European troops an exit strategy.
"There are no dates because we'll set them with the upcoming Iraqi government," Berlusconi was quoted as saying.
Poland, which has about 2,500 troops in Iraq, on Tuesday handed over authority for Karbala province, southwest of Baghdad, to the 11th Marine Expeditionary Unit. The U.S. military described the handover as part of a "realignment of forces in southern Iraq."
In Hit, a town halfway between Baghdad and the Jordanian border in restive Anbar province, Marines called in an AC-130 Spectre gunship to target 10 insurgents who fired on Marines conducting a series of raids in the town. The heavily armed propeller-driven plane "silenced the threat," the Marines said in a statement.
Mosul, to the north, has been the scene of almost daily firefights since early last month, when insurgents shocked local commanders by overrunning the city's police stations in groups of as many as 60. Since then, more than 150 bodies have been found in the city, many of them Kurdish members of federal security forces summoned to help the resident U.S. force restore order.
As Army investigators picked through the mess hall for clues to the origin of the blast, the looming question was whether insurgents had been able to smuggle a powerful explosive onto the base, or if an insurgent mortar crew had managed an accurate shot.
Mortar rounds fall frequently on the post -- sometimes a half-dozen a day. This week, one insurgent group, the al Mustafa Brigade, boasted of firing 15 60mm mortars toward the Marez base, posting video of men in ski masks manning the tubes.
Most of the time, the explosions are shrugged off by soldiers as little more than a nuisance. Most are fired quickly and at random by insurgents who leap from cars in the city's busy streets without taking necessary measurements.
When mortars do strike buildings on the post, the information is usually kept secret to avoid tipping off attackers about the accuracy of their strikes. U.S. officers worry, however, that insurgent informants on the post may be passing targeting data to attackers on the outside to help them refine their fire.
The Sunni-led insurgency in the city operates on the edge of the post boundaries. One battalion commander's convoy was struck by a roadside bomb, followed by an ambush just outside the main gate this month.
Correspondent Scott Wilson in Dubai, United Arab Emirates, and staff writer Thomas E. Ricks in Washington contributed to this report.