A police chief was killed Monday by a bomb planted in his office in a southern Iraqi town that saw heavy clashes last month between government forces and the Shiite militia of cleric Muqtada al-Sadr.

Also Monday, Iraqi security forces launched raids in Shiite militia strongholds in the city of Basra after gunmen killed one policeman and wounded three others.

The violence was the latest to shake fragile truces between Shiite gunmen and the government that eased widescale clashes that erupted in early April over a crackdown on militiamen in Basra. The south and the Shiite district of Sadr City in Baghdad have continued to see low-level clashes for weeks amid arrests of Shiite fighters.

Monday's bombing killed Lt. Col. Farhan Qassim, chief of police in Suq al-Shiyoukh, an area outside Nasiriyah, 200 miles southeast of Baghdad. The blast went off inside Qassim's office as he entered it in the morning, police in Nasiriyah said.

Suq al-Shiyoukh was the scene of heavy fighting on April 19 between police and members of al-Sadr's Mahdi Army that left 22 people dead.

The area has also seen apparent infiltration of police by Shiite militiamen. A week ago, a bomb detonated in the province's main police command in Nasiriyah, wounding two officers. Four policemen were arrested soon after.

Further south, Iraqi solders and police launched pre-dawn raids in four neighborhoods of Basra, including two Shiite militia enclaves, arresting several suspects, Basra's operations command Maj. Gen. Moham