MOSUL, Iraq, Dec 21 (Reuters) - A guerrilla attack on a dining tent at a U.S. base in Iraq killed 19 American soldiers and three other people on Tuesday in the deadliest strike on U.S. forces since last year’s war to oust Saddam Hussein.

U.S. officials initially said a number of rocket and mortar rounds were fired at the base in the northern city of Mosul, but a militant group claimed one of its suicide bombers was behind the attack, in which 57 people were wounded.

"A suicide bomber has not been ruled out," a U.S. Army official said later in Washington. Lieutenant Colonel Steve Boylan, chief spokesman in Baghdad, said 19 American soldiers were among the 22 people killed.

U.S. authorities said the three others killed were not U.S. citizens but did not give their nationalities.

"We send our heartfelt condolences to the loved ones who suffered today. We just want them to know that the mission is a vital mission for peace," said U.S. President George W. Bush, who warned on Monday guerrilla bombers were taking a toll.

"I’m confident democracy will prevail in Iraq. I know a free Iraq will lead to a more peaceful world,"said Bush, who paid a Christmas visit to families of wounded soldiers as a poll showed most Americans now believed the war was not worth fighting.

The attack was a reminder of the threat guerrillas pose, six weeks after U.S. troops stormed Falluja to quell a Sunni Arab insurgency before Iraq holds national elections on Jan. 30.

The U.S. military deaths surpassed the toll of 17 when two helicopters came down in Mosul in November last year.

The attack occurred as Bush’s strongest ally on Iraq, British Prime Minister Tony Blair, made a surprise visit to Baghdad, vowing the guerrillas would be defeated and that the elections would go ahead.

The Mosul attack occurred at noon (0900 GMT) when U.S. soldiers at Forward Operating Base Marez, a huge camp built round the city’s airfield, were sitting down to lunch four days before Christmas in a vast tented hall made of canvas and metal.

"A fireball enveloped the top of the tent, and pellet-sized shrapnel sprayed into the men," wrote witness Jeremy Redmon, a journalist for the U.S. Richmond Times-Dispatch newspaper.

"Amid the screaming and thick smoke that followed, quick-thinking soldiers turned their lunch tables upside down, placed the wounded on them and gently carried them into the parking lot."

Mosul, an ethnically mixed city of Arabs and Kurds, was seen as a U.S. success story until last month when guerrillas routed the U.S.-trained police as U.S. troops were concentrating on storming Falluja, west of Baghdad. Mosul has seen near anarchy since; possibly hundreds have been killed, most of them Iraqis.

U.S. commanders say Jordanian al Qaeda ally Abu Musab al-Zarqawi probably fled Falluja before the U.S. offensive and may have shifted his base to Mosul, Iraq’s third city.

Iraqi militant group Ansar al-Sunna, a known force in the 18-month insurgency, said a suicide bomber was behind the blast.

Little can stop the odd deadly strike with a mortar. But a bomber infiltrating a camp would have grave implications for bases where U.S. forces work with fledgling Iraqi forces who they hope can one day take over and let the Americans go home.

Blair, who has twice visited calmer southern Iraq but never seen Baghdad, flew in to meet Iraqi Prime Minister Iyad Allawi in the city’s Green Zone compound, which houses the Iraqi government and U.S. military. He arrived under tight security.

Hailing Iraq’s election workers as "heroes", Blair launched a passionate defence of the war as vital for Britain’s security and Iraqis’ freedom.

"Here are people who are risking their lives every day in order to make sure that the people of Iraq get a chance to decide their own destiny," Blair told a news conference after meeting Election Commission chiefs running Iraq’s Jan. 30 poll.

Three Commission employees were killed in Baghdad on Sunday.

Blair, who later met British troops in Basra, was clearly

aware of the threats in Iraq’s capital, where there are daily shootings and bombings, not least on the Green Zone, a former palace complex belonging to Saddam.

"You can feel the sense of danger people live in ... I feel a sense of humility," he said, turning to Allawi. "It’s a very tough challenge you face. You feel the sense of the challenge."

Both Blair and Allawi portrayed the fight against insurgents in Iraq, most of whom are Saddam loyalists or Sunni Muslim militants, as part of the U.S. administration’s war on terrorism launched after Sept. 11, 2001.

Two French journalists, held hostage for four months were freed, ending a saga that had embarrassed the Paris government.