BAGHDAD, Iraq — A 122mm rocket slammed into a mess tent Tuesday at a military base near the northern city of Mosul, ripping through the ceiling and spraying shrapnel as U.S. soldiers sat down to lunch. Officials said 22 people were killed in one of the most devastating attacks against Americans in Iraq since the start of the war.

The dead included 20 Americans — 15 servicemembers and five civilian contractors. Two Iraqi soldiers also were killed. Sixty-six people were wounded, including 42 U.S. soldiers, Capt. Brian Lucas, a military spokesman in Baghdad, said early Wednesday.

Inside the tent, U.S. 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 Jeremy Redmon, a reporter for the Richmond (Va.) Times-Dispatch embedded with the troops in Mosul.

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 Jan. 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 sleeping 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 British Prime Minister Tony Blair made a surprise visit to Baghdad and spoke of a "battle between democracy and terror."

White House spokesman Scott McClellan, responding to a question about how Iraqis will be able to safely get to some 9,000 polling places if U.S. 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. But insurgent attacks in the largely Sunni area have increased dramatically in the past year — particularly since the U.S.-led military offensive in November to retake Fallujah from militants.

Like most mess halls at U.S. 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, Redmon said.