Updated

Iraqi soldiers for the first time warned residents in the embattled Sadr City district to leave their houses Thursday, signaling a new push by the U.S.-backed forces against Shiite extremist who have been waging street battles for seven weeks.

Iraqi soldiers, using loudspeakers, told residents in some areas of southeastern Sadr City, which were virtually abandoned, to go to nearby soccer stadiums, residents said. UNICEF says about 6,000 people have been forced to flee their homes in Sadr City, most of them from the southeastern section.

U.S. forces have increased air power and armored patrols in an attempt to cripple Shiite militia influence in Sadr City, a slum of 2.5 million people that serves as the Baghdad base for the Mahdi Army led by anti-American cleric Muqtada al-Sadr.

The U.S. military is trying to weaken the militia grip in the slum and disrupt rocket and mortar strikes from Sadr City on the U.S.-protected Green Zone, which includes the U.S. Embassy and key Iraqi government offices.

Several civilians were injured in rocket or mortar attacks that hit downtown Baghdad earlier this week.

The battles started in late March after the Iraqi government opened a crackdown on militias and armed gangs in the southern city of Basra, including some groups Washington says have links to Iran.

Meanwhile, a former leader of Al Qaeda in Iraq, Mulla Nadhim al-Jibouri, was wounded in a suicide attack in the city of Samarra, about 60 miles north of Baghdad, on Thursday, Iraqi police said.

The attack came after Iraqi and U.S. soldiers killed five Sunni insurgents who belonged to al-Qaida in Samarra overnight, police said.

In Sadr City, at least seven people were killed and 20 wounded in clashes late Wednesday, Iraqi health officials said Thursday. It was not clear whether any militants were among them.

On Wednesday, Claire Hajaj, a UNICEF spokeswoman based in Jordan, said up to 150,000 people — including 75,000 children — were isolated in sections of Sadr City "cordoned off by military forces." She said about 6,000 have fled their homes.

Iraqi soldiers on Thursday shut down a local radio station, al-Aahad, run by the Sadrists after raiding offices of the station in a neighborhood near Sadr City, police said.

Gunmen wearing police uniforms killed a police captain Wednesday after kidnapping him from a police station in Shiite neighborhood of Abu Dshir, police said Thursday. His body was found near the station.

In the southeastern Shiite city of Kut, gunmen on Thursday stormed the Technical Institute and abducted a professor, Nuri Kamil Khanjar, local police said. It was not clear why Khanjar was kidnapped.