Iraqi insurgents have attacked several U.S. military dining hall tents in recent months and the Pentagon was finishing a hardened bunker to replace the dining tent at the base near Mosul (search) attacked Tuesday, military officials said.

Days before the hardened dining hall was scheduled to be completed, a 122mm rocket slammed into the tent at Forward Operating Base Marez (search) near Mosul where hundreds of troops were sitting down to lunch.

Accounts of the number killed varied, but Pentagon officials said Tuesday night that 22 were killed, including 20 Americans. The toll included 15 military servicemembers and five civilians. More than 60 were wounded, including U.S. troops, civilian workers and Iraqi soldiers.

A spokeswoman for Halliburton (search), the Army contractor which provides food services through its KBR subsidiary, said four company employees and three subcontractors were killed. Safety is a concern for the contract workers, as well.

"It is extremely difficult to prevent these appalling and horrific attacks," Halliburton spokeswoman Wendy Hall said Tuesday.

Attacks from rockets or mortars — what the military calls "indirect fire" — have been commonplace for months at U.S. bases in the Mosul area as well as other insurgency hot spots in Iraq. Dining halls are a prime target because they offer a readily identifiable place where lots of troops congregate at predictable times.

For example, a mortar round hit near the mess hall of a U.S. base in Tikrit during dinner one night in March. The round didn't explode and no one was injured. Insurgents also launched rockets that month which exploded near a large military dining hall within Baghdad's Green Zone where U.S. and Iraqi government offices are located. Another mortar round injured three soldiers at a dining hall on another Baghdad base in February.

At many bases — including Marez — troops have been required to wear their body armor and helmets while in the dining hall because of the threat of attack. Most of the attacks don't hit any structures or cause any injuries, however.

The military was building a bunker-like mess hall at the Marez base to protect against such indirect fire attacks, Defense Department officials said. The new dining hall was part of continuing efforts to make the base safer, said Lt. Col. Paul Hastings, a spokesman for the U.S. military command in Mosul.

"There is a level of vulnerability when you go in there and you don't feel like there's a ... hard roof over your head," Hastings said in a televised interview.

One soldier was killed near the dining hall at Marez in a mortar attack in May, and two soldiers were killed in November when mortars exploded in their living area on the same base.

Maj. John Nelson, the battallion's chief surgeon, told a reporter earlier this year about plans for a possible attack on the dining hall. Nelson told the Portland (Maine) Press Herald that military statistics showed that if a 60mm mortar shell hit the dining hall with 400 soldiers inside, an estimated 12 would die no matter what medics could do.

Meanwhile, President George W. Bush said Tuesday's deadly attack should not derail elections scheduled for next month and he hoped relatives of those killed would find solace in the service their loved ones provided.

"We just want them to know that the mission is a vital mission for peace," Bush said.

Bush said the violence, part of a continuing wave of unrest in Iraq, should not affect elections scheduled for Jan. 30.

"I'm confident democracy will prevail in Iraq," he said.

White House spokesman Scott McClellan, responding to a question about how Iraqis will be able to get to some 9,000 polling places for the elections if U.S. troops can't secure their own bases, said there was "security and peace" in 15 of Iraq's 18 provinces.

"There are tough challenges that remain ... but we are making important progress on the ground," McClellan said. "We also have to keep in mind that the terrorists and Saddam loyalists have adapted and changed their tactics. We adapt and change with that as well to meet those ongoing security challenges."

Both American and Iraqi forces use the base attacked Tuesday. A surge in killings and other attacks in Mosul in recent weeks has targeted members of the Iraqi security forces in particular, with the bodies of many Iraqi soldiers found dumped in the streets as a warning to others.

Halliburton subsidiary KBR has gotten more than $8 billion (euro5.97 billion) worth of work supporting U.S. forces in Iraq, performing functions such as building and maintaining housing, washing clothes, delivering supplies and serving food. As on the base attacked Tuesday, KBR typically runs the mess halls in cavernous tents, which include cafeteria-style serving lines as well as tables piled with fresh fruit, soft drinks and pastries.

Hall said the deaths in the Mosul attack bring to 62 the number of Halliburton workers or subcontractors killed in and around Iraq.

"These brutal attacks are unsettling, appalling and very sad for everyone," Hall said in a statement. "We are doing everything we can to assist the people on the ground."

The identities of the attackers were unknown, though a radical Muslim group later claimed responsibility.