Twenty-two people, including 19 US soldiers, have been killed in an explosion at a US military base in the northern Iraqi city of Mosul.
One US official said it was a mortar or rocket strike but 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 a suicide bomber was behind the blast. The military said it was investigating the cause.
The attack came as British Prime Minister Tony Blair made a surprise visit to Baghdad where he vowed the war on insurgents would be won and elections would go ahead on January 30th.
As he left, mortars fell on Baghdad's Green Zone compound, as they do almost daily. There appeared to be no 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, were sitting down to lunch.
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.
Mosul, Iraq's third largest city, has seen ongoing violence for the past few weeks since Sunni Arab insurgents routed the US-trained police force in November while many US forces were concentrated on storming militant bases in Fallujah.
Ethnic tensions have also been inflamed in a city that is home to both Arabs and Kurds. There has been speculation that Jordanian al-Qaeda ally Abu Musab al-Zarqawi, once active around Fallujaf, west of Baghdad, had moved some of his operations to the Mosul area.
The city, 240 miles north of Baghdad, was home to some loyalists of Saddam Hussein's old regime. Saddam's two elder sons were hiding there when they were surrounded and killed by US troops in July last year.
Gunmen have roamed parts of Mosul with ease in recent weeks. Last week, five Turkish security guards from Ankara's embassy in Baghdad and two of their Iraqi drivers were killed when a convoy was ambushed in the northern city.