A man examines an ambulance destroyed in an apparent U.S. airstrike Saturday in the Shiite stronghold of Sadr City in Baghdad. The strike wounded nearly two dozen people.

Baghdad ? The U.S. military fired guided missiles into the heart of Baghdad’s teeming Sadr City slum on Saturday, leveling a building 55 yards away from a hospital and wounding nearly two dozen people.

Separately, the U.S. military said late Saturday that four Marines were killed on Thursday by a roadside bomb in Fallujah province. The military also said that a U.S. soldier died of wounds suffered in a roadside bomb that struck the soldier’s vehicle during a combat patrol in eastern Baghdad on Friday. At least 4,071 members of the U.S. military have died since the beginning of the Iraq war in March 2003, according to an Associated Press count.

AP Television News footage from Sadr City showed several ambulances destroyed and on fire, thick black smoke rising from them as firefighters worked to put out the flames.

The strike, made from a ground launcher, took out a militant “command-control center,” the U.S. military said. The center was in the heart of the eight-square-mile neighborhood that is home to about 2.5 million people. Iraqi officials said at least 23 people were wounded, though none of them were patients in the hospital.

The U.S. military blamed the militants for using Iraqi civilians as human shields.

“This is a circumstance where these criminal groups are operating directly out of civilian neighborhoods,” military spokeswoman Spc. Megan Burmeister told The Associated Press in an e-mail.

She said it presents a “complex and very difficult” challenge for U.S. forces to strike the militants when they are “putting themselves next to municipal buildings.”

Dr. Ali Bustan al-Fartusee, director general of Baghdad’s health directorate, told the AP that 23 civilians were wounded in the strike.

He said no patients in the hospital were hurt, but that some of the wounded included civilians outside on their way to visit patients in the hospital. He also said 17 ambulances were damaged or destroyed.

AP Television News footage showed about 100 people milling about in the rubble of the destroyed building. A deep crater was seen just yards from the hospital, which is surrounded by 15-foot-tall concrete blast walls. It appeared that one section of the blast wall was leveled.

Windows were blown out of cars in the hospital’s parking lot, but there did not appear to be any damage to the hospital itself.

Shiite extremists are known to have operated in a building next to the hospital, local reporters said.

U.S. and Iraqi forces have waged street battles with Shiite militias since late March in Sadr City, the power base of radical Shiite cleric Muqtada al-Sadr and his Mahdi Army militia.

The fighting is part of a 5-week-old crackdown by the Iraqi government and U.S. forces on Shiite militia factions. The clashes have brought deep rifts among Iraq’s Shiite majority and have pulled U.S. troops into difficult urban combat.

Militia members have been blamed for firing hundreds of rockets or mortars from Sadr City into the Green Zone, the U.S.-protected area housing the American embassy and much of the Iraqi government. In the past month, more than a dozen people – including two American civilians and two U.S. soldiers – have been killed inside the zone during the attacks.

In response to the shelling, American and Iraqi troops in recent weeks have moved into Sadr City, hoping to push the militants far enough from the Green Zone so their rockets and mortars would be out of range.