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BAGHDAD, Iraq - A devastating explosion tore through the dining tent of a U.S. military base at lunchtime Tuesday, killing at least 22 people, including 15 American soldiers, and wounding about 60 people in the volatile northwest city of Mosul.

It was the single deadliest attack on U.S. troops since the war began.

Also among the casualties were American security contractors, Iraqi security personnel and other foreign nationals working at Forward Operating Base Marez, according to the U.S. military. The base, about 220 miles north of the capital, is used by American troops and the interim Iraqi government's security forces.

The attack on the base was the latest in the insurgents' campaign to sabotage the Jan. 30 parliamentary elections by keeping large swaths of the country unstable. With the attack in Mosul, where insurgents regularly hit Iraqi police and national guard sites, and two powerful bombings this week in Shiite Muslim holy cities, rebels proved they can still strike at will from the north to the south.

The militant group Ansar al-Sunnah Army claimed responsibility for the attack in an Internet statement that described it as a ''martyrdom operation.'' The same group was behind the executions of 12 Nepalese workers and claimed responsibility for other violence in the Mosul area.

The explosion ripped through the flimsy tent roof of the mess hall just as hundreds of soldiers sat down for lunch, according to the military and an account provided by an American reporter embedded with the troops. A sturdier dining hall with a reinforced roof was reportedly under construction.

After first blaming the devastation on rockets or mortars, military officials said the cause of the blast was still under investigation and called it ''a single explosion.''

President Bush said Tuesday that he was saddened by the deaths in Mosul, noting that it ''is particularly sorrowful for the families as we head into the Christmas season. We pray for them. We send our heartfelt condolences to the loved ones who suffer today.''

He called the war ''a very important and vital mission.''

''I'm confident democracy will prevail in Iraq. I know a free Iraq will lead to a more peaceful world,'' Bush said after he visited soldiers at Walter Reed Army Medical Center in Washington.

Insurgent activity increased in Mosul, Iraq's third-largest city, during last month's U.S.-led offensive on the formerly rebel-held city of Fallujah. Earlier on Tuesday, hundreds of students demonstrated at the center of Mosul, demanding that U.S. troops cease raids on homes and mosques. Iraqi forces also prevented an attack by insurgents trying to seize a police station at the center of the city.

On Sunday, rebels launched three separate attacks on U.S. forces with homemade explosives and a car bomb.

From the Sunni town of Fallujah to the Shiite slum of Sadr City, residents drew distinctions between an attack on American forces, widely viewed as occupiers, and terrorism that kills civilians.

''Don't you consider all the actions conducted against us by the Americans as terrorism?'' asked Talaat al Wazan, secretary general of the Iraqi National Unity political party.

Sheik Hassan al Athari, a Baghdad representative of the rebel Shiite cleric Muqtada al-Sadr, said militants were exercising their right of self-defense in the face of foreign occupation. He said such attacks are to be expected ''as long as Iraq remains a laboratory for the experiments of American forces as well as foreign terrorists.''