Iraqi police arrested 31 Shi'ite activists Saturday in early morning raids south of Baghdad, and five American soldiers were killed in two roadside bombings, officials said. The US troops were killed Friday - four in Baghdad and one in the northern Tamim province, the military said. At least 3,958 members of the US military have died since the beginning of the Iraq war in March 2003, according to an Associated Press count. Saturday marked a third day of US and Iraqi operations in an area that includes several Shi'ite holy cities - raising tension with some Shi'ite tribesmen and fighters who have pledged to halt attacks. Radical Shiite cleric Muqtada al-Sadr ordered a six-month cease-fire for his Mahdi Army militia, but some members have broken away and violated the pledge which expires later this month.