Attack on US military dining hall kills 24
The attack came as British Prime Minister Tony Blair made a surprise visit to Baghdad, where he vowed the war against insurgents would be won and elections would go ahead on January 30.
As he left Baghdad, mortars fell on the Green Zone compound, as they do almost daily. There was no word on any casualties.
The Mosul strike came at noon when many soldiers at Forward Operating Base Marez, a huge camp built around the northern city's airfield, would have been eating lunch. The tented dining hall can seat hundreds of soldiers at a time, reporters who have stayed at the base said.
Major General Carter Ham, the commander of the 8,000 US troops based in Mosul, said the dead included US soldiers, US and foreign contractors and members of the Iraqi army.
"More than 20 have been killed and more than 60 people have been wounded," he said, adding there was a single explosion.
In the bloodiest previous single incident for US troops in Iraq, two Black Hawk helicopters crashed in Mosul in November last year, killing 17 soldiers. At the start of the war in March last year, 29 soldiers were killed in a fierce day of fighting.
Iraqi militant group Ansar al-Sunna, a known Sunni Muslim faction that has been at the heart of the 18-month insurgency against US forces, said it was behind the attack.
Responding to the attack, the White House vowed that the "enemies of freedom" would be defeated. On Monday, US President George Bush warned that Iraqi bombers were having an impact.
Mosul has seen a surge in violence over the past six weeks, since US forces launched an offensive against insurgents holed up in Fallujah, an assault designed to break the back of the guerrilla movement operating in the country.
Mr Blair, who has visited Iraq twice but never Baghdad, flew to meet Iraqi Prime Minister Iyad Allawi in the city's Green Zone compound, which houses the Iraqi government and US military, arriving under tight security.
Hailing Iraq's election workers as "heroes", Blair, who is expected to call an election next year, 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 January 30 poll.
Three people working for the Commission were killed by gunmen in Baghdad two days ago.
"You can feel the sense of danger people live in," he said, turning to Mr Allawi. "It's a very tough challenge you face."
But Mr Blair said he had no doubt Britain was right to have helped oust Saddam.
Both Mr Blair and Mr Allawi were at pains to portray the fight against insurgents in Iraq, most of whom are loyalists to the former regime or Sunni Muslim militants, as part of the US administration's war on terrorism launched after September 11, 2001.
"We stand on the side of the democrats against the terrorists," Mr Blair said.





