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. No other details were released and the names of the Marines were withheld pending notification of their families.

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 located 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.