BAGHDAD, Iraq – A 122mm rocket slammed into a mess tent yesterday at a military base near the northern city of Mosul, ripping through the ceiling and spraying shrapnel as American soldiers sat down to lunch. Officials said 22 people were killed in the deadliest single attack against Americans in Iraq since the start of the war.

The dead included 20 Americans – 15 of them service members and five civilian contractors. Two Iraqi soldiers also were killed. Sixty-six people were wounded, including 42 American troops, Captain Brian Lucas, a military spokesman in Baghdad, said today.

Inside the tent, American soldiers reacted quickly. With people screaming and thick smoke billowing, soldiers turned their lunch tables upside down, placed the wounded on them, and gently carried them into the parking lot, said a reporter for the Richmond (Va.) Times-Dispatch embedded with the troops in Mosul, Jeremy Redmon.

A radical Sunni Muslim group, the Ansar al-Sunnah Army, claimed responsibility for the attack – the latest in a week of deadly strikes across Iraq that highlighted the unwavering power of the insurgents in the run-up to the January 30 national elections.

President Bush said the explosion should not derail the elections and that he hoped relatives of those killed know that their loved ones died in “a vital mission for peace.”

“I’m confident democracy will prevail in Iraq,” he said.

Portland (Maine) Press Herald photographer Gregory Rec, who was about a quarter-mile from the mess hall when he was awakened by the loud explosion, said he rushed to the scene, where a soldier told him “he heard a whoosh, he looked up and saw a fireball halfway between the ceiling and the floor.”

The attack at Forward Operating Base Marez came hours after Prime Minister Blair made a surprise visit to Baghdad and spoke of a “battle between democracy and terror.”

White House spokesman Scott Mc-Clellan, responding to a question about how Iraqis will be able to safely get to some 9,000 polling places if American troops can’t secure their own bases, said there was “security and peace” in 15 of Iraq’s 18 provinces.

Mosul, Iraq’s third-largest city, was relatively peaceful in the immediate aftermath of the fall of Saddam Hussein’s regime last year. But insurgent attacks in the largely Sunni area have increased dramatically in the past year – particularly since the American-led military offensive in November to retake Fallujah from rebels.

Like most mess halls at American bases in Iraq, the dining area at Base Marez is covered with a tent. Insurgents have fired mortars at the mess hall more than 30 times this year, Mr. Redmon said.

Mortar attacks on American bases, particularly on the huge white tents that serve as dining halls, have been frequent in Iraq for more than a year. Just last month, for example, a mortar attack on a Mosul base killed two troops with Task Force Olympia, the reinforced brigade responsible for security in much of northern Iraq.

Bill Nemitz, a columnist with the Portland (Maine) Press Herald who was embedded with the troops in Mosul, told CNN that he heard “a lot of discussion” about the vulnerability of the tent.

Lieutenant Colonel Paul Hastings, a spokesman for Task Force Olympia, acknowledged the tent’s vulnerability and told CNN the military is building a new dining facility at the base – a concrete structure that Mr. Nemitz said was supposed to have been ready for Christmas.

“There is a level of vulnerability when you go in there and you don’t feel like there’s a…hard roof over your head,” Lieutenant Hastings told CNN.

Base Marez, also known as the al-Ghizlani military camp, is three miles south of Mosul and is used by both American troops and the interim Iraqi government’s security forces. It once was Mosul’s civilian airport but is now a heavily fortified area surrounded by blast walls and barbed wire. Its two main gates are guarded by American troops; Iraqi National Guard members man checkpoints outside to prevent cars from getting close without being searched.

Earlier, Brigadier General Carter Ham, commander of Task Force Olympia, said American military personnel, American and foreign nationals, and Iraqi soldiers were among the dead. “It is indeed a very, very sad day,” General Ham said.

Mr. Redmon said the dead included two soldiers from the Richmond based 276th Engineer Battalion, which had just sat down to eat. The force of the blast knocked soldiers off their feet and out of their seats as a fireball enveloped the top of the tent and shrapnel sprayed into the area, Mr. Redmon said.

Scores of troops crammed into concrete bomb shelters, while others wandered around in a daze and collapsed, he said.

“I can’t hear! I can’t hear!” one female soldier cried as a friend hugged her.

A huge hole was blown in the roof of the tent, and puddles of blood, lunch trays, and overturned tables and chairs covered the floor, Mr. Redmon reported.

Near the front entrance, troops tended to a soldier with a serious head wound, but within minutes, they zipped him into a black body bag, he said. Three more bodies were in the parking lot.