The allied air campaign, credited with decimating Iraq's best defenders of Baghdad, is shifting its focus to targets mostly in northern Iraq while backing up Army and Marine ground troops in Baghdad and elsewhere, officials said Thursday.

The overall pace of the air campaign has not slackened, even with the fall of the Iraqi capital, but there are fewer targets to strike in areas that have come under American or British control.

Defense officials said some Navy and Marine Corps attack planes, unable to find enough targets, are returning from combat missions with some unused bombs.

U.S. planes flew about 1,750 missions on Thursday, including about 550 strike missions.

The numbers have held steady over the past week at about 1,700 total and 500-600 combat missions a day. About 80 percent of Thursday's missions were in support of U.S. ground forces in Baghdad and U.S. special operations forces in northern and western Iraq, officials said.

Those figures do not include helicopter missions, which have figured prominently in the war in attack and support roles. On Thursday, for example, the 101st Aviation Brigade and other elements of the Army's 101st Airborne Division, equipped with Apache attack helicopters and other weaponry, destroyed 41 tanks and dozens of other armored vehicles and artillery of Republican Guard units south of Baghdad, officials said.

At the Pentagon, Maj. Gen. Stanley McChrystal, vice director of the Joint Staff, told reporters that Iraqi ground forces in the Mosul and Tikrit areas in northern Iraq are a major focus of allied airstrikes. He said those Iraqi forces are the last major ground units still fighting in Iraq.

The Pentagon, meanwhile, said the number of Americans killed in the Iraq war since it began March 20 rose to 105 and the missing stood at 11. Seven are listed as prisoners of war, and McChrystal said he had no information he could disclose publicly about efforts to find the seven POWs.

The number of Americans wounded in action stood at 343, the Pentagon said.

The Pentagon provides no figures on Iraqi casualties.

McChrystal showed gun-camera video from airstrikes against three targets in the Tikrit area. One target was a military complex of the Special Republican Guard, the forces most loyal to Saddam Hussein. It included a barracks, a small-arms firing range and a military parade field.

Also targeted was what McChrystal called a retreat house used by government leaders, and a facility that was used to jam radio broadcasts of foreign news services and of Iraqi opposition groups.

Those three targets were struck between April 3 and Wednesday.

"Now that there's less of the regime power available, we've been able to focus more at certain areas like the units around Tikrit or the regular army divisions in the north, which allows us to do more damage," he said.

In addition to Marine Corps and Air Force planes flying from bases in and around Iraq, the Navy is flying aircraft off five aircraft carriers -- two in the eastern Mediterranean Sea and three in the Persian Gulf. The total number of aircraft, including support planes, is about 2,000.

Although there are fewer strikes now against Iraqi government structures in Baghdad, warplanes are still on call to deliver the kind of short-notice strike that a B-1B bomber conducted Monday when U.S. intelligence identified a building where it believed the Iraqi president was meeting with other government leaders.