"At 12 p.m. today an explosion occurred at a U.S. military installation in Mosul causing multiple casualties," a military statement said. "The cause of the explosion is under investigation."
At the Pentagon, a Defense Department official speaking on condition of anonymity said the rocket and missile attack, which was believed to have hit a dining hall, killed 22 people and injured 50.
While military officials were unable to confirm whether the casualties were U.S. or Iraqi personnel, sources told CBS News that U.S. military and civilian contractors, along with Iraqi national guardsmen were among the casualties. The base is used jointly by the U.S. military and the interim Iraqi government's security forces.
Witnesses said the shelling occurred at the Al Ghizlani military camp, about 3 miles south of Iraq's third-largest city.
The Ansar al-Sunnah Army claimed responsibility for the attack in a statement published on the Internet.
Ansar al-Sunna is believed to be a fundamentalist group whose goal is to turn Iraq into a tightly controlled Islamic state like Afghanistan's former Taliban regime. In August, the Sunni Muslim group claimed responsibility for the beheading of 12 Nepalese hostages.
In other developments:
Earlier in the day, hundreds of students demonstrated in the center of the predominantly Sunni Muslim city, demanding that U.S. troops cease breaking into homes and mosques in the city.
Also Tuesday, Iraqi security forces repelled another attack by insurgents as they attempted to seize a police station in the center of the city, the U.S. military said in a statement.
"An Iraqi police station came under attack by indirect and small arms fire during a coordinated effort by insurgent fighters to overrun the station in central Mosul," the statement said. "The Iraqi police successfully repelled the attack."
Mosul was relatively peaceful in the immediate aftermath of the fall of Saddam Hussein's regime last year. But insurgent attacks have increased dramatically in the past year and particularly since the U.S.-led military operation in November to retake the restive city of Fallujah from guerrillas.
Since Nov. 10, about 160 people in and around Mosul have been murdered, including many affiliated with the Iraqi National Guard and police.
Mosul was the scene of the deadliest single incident for U.S. troops since the attack on Iraq last year. On Nov. 15, 2003, two Black Hawk helicopters collided in mid-air over the city, killing 17 soldiers and injuring five. The crash occurred as the two choppers maneuvered to avoid ground fire from insurgents.
Earlier Tuesday, British Prime Minister Tony Blair made a surprise visit to Baghdad and described violence there as a "battle between democracy and terror."
Blair held talks with Interim Prime Minister Ayad Allawi and Iraqi election officials, who he called heroes for carrying out their work despite attacks by insurgents. Three members of Iraq's election commission were dragged from the car and killed this week in Baghdad.
"I said to them that I thought they were the heroes of the new Iraq that's being created, because here are people who are risking their lives every day to make sure that the people of Iraq get a chance to decide their own destiny," Blair said during a joint news conference with Allawi.
Blair, whose trip to Iraq hadn't been disclosed for security reasons, urged Iraqis to back the Jan. 30 national vote, the first since Saddam Hussein's ouster.
"Whatever people's feelings and beliefs about the removal of Saddam Hussein, and the wisdom of that, there surely is only one side to be on in what is now very clearly a battle between democracy and terror," said.
The British leader said that apart from the insurgents' violence, "there is another choice for Iraq: the choice is democracy, the choice is freedom, and our job is to help them get there because that's what they want."
Allawi said his government was committed to holding the elections as scheduled next month, despite calls for their postponement owing to the violence.
"We have always expected that the violence would increase as we approach the elections," Allawi said. "We now are on the verge, for the first time in history, of having democracy in action in this country."
Blair said that as the U.S.-led multinational force, in which British troops are serving, trains and improves the Iraqi security forces, "that brings forward the day that the multinational force can leave" Iraq. The presence of foreign troops in Iraq is strongly opposed across the Arab world.
It was Blair's first visit to Baghdad and his third to Iraq since the dictator Saddam Hussein was toppled in April 2003. Blair visited British troops stationed around the southern Iraqi city of Basra in mid-2003 and in January. President Bush had paid a late night visit to U.S. troops in Baghdad in November 2003.