US and Iraqi troops exchanged gunfire Sunday with Sunni militants in central Baghdad in a second day of clashes following the arrest of a local leader of Sunni security volunteers who had broken with al-Qaida. Four people were killed and 15 wounded Saturday when fighting broke out after police arrested Adil al-Mashhadani, the head of an Awakening Council group in the Sunni neighborhood of Fadhil in the heart of the capital. Five Iraqi soldiers were missing, possibly captured by Awakening Council fighters, a police officer said on condition of anonymity because he was not supposed to release the information. Fighting tapered off Saturday evening. But an Associated Press photographer and a local shopkeeper said clashes erupted again Sunday morning, with both US and Iraqi troops firing at Sunni gunmen. There was no immediate report of casualties.