In the northern town of Beiji, meanwhile, a mortar fell on a government facility that Deputy Prime Minister Ahmed Chalabi was visiting Wednesday, an aide said. Chalabi was not harmed and later returned to Baghdad, the aide said on condition of anonymity because he was not authorized to release the information. Chalabi, who is also the interim oil minister, was believed to have been visiting the refinery in Beiji, the nation's largest.

As U.S. officials step up pressure on Iraqi leaders to form a national unity government quickly, the United States' top military commander said he had underestimated the extent of Iraqi reluctance to come together.

"I think that I certainly did not understand the depth of fear that was generated by the decades of Saddam's rule," said Gen. Peter Pace, chairman of the Joint Chiefs of Staff. "I think a lot of Iraqis have been in the wait-and-see mode longer than I thought they would."

Pace said one solution was for the Iraqis to do a better job of recruiting more Sunnis into the army and for police forces to balance Shiite domination.

"A unit that has all (sects of) Iraqis embedded in it is better able to handle whatever kind of strife comes along," the general said.

The Bush administration views formation of a broad-based government as a first step in quelling violence and allowing the start of an American troop withdrawal this summer.

While the U.S. military has touted its progress in training the Iraqi army and police, a top expert on Iraq said the forces remained poorly matched against the insurgency and al-Qaida.

"The police have almost no protected vehicles, few heavy weapons similar to those of insurgents, are often located in extremely vulnerable buildings, and have weak communications. Corruption is a major issue," Anthony H. Cordesman, of the Center for Strategic and International Studies, wrote in a position paper released this week.