Iraqi police arrested 31 Shiite 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 Shiite holy cities - raising tension with some Shiite 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. US and Iraqi forces say they are targeting rogue, criminal elements of his and other militias. But several Shiite imams, during Friday prayers, suggested Iraqi forces were taking advantage of the cease-fire to crack down on rival groups. Al-Sadr has threatened not to extend his cease-fire unless the government purges rival Shiite militiamen he alleges have infiltrated the security forces and are targeting his followers. Fifteen of Saturday's arrests took place in Karbala, a Shiite holy city 80 kilometers south of Baghdad, where Shiite Islam's two most revered saints are buried. Another 16 men were arrested in a Sadrist area of Nasiriyah, about 320 kilometers southeast of the capital, police said. Rahman Mshawi, spokesman for Karbala police, said four of the Karbala suspects are members of the Iraq-based People's Mujahedeen Organization of Iran, or Mujahedeen Khalq. The group was founded in the late 1960s and fled to Iraq in the early 1980s after it fell out with the clerical regime of Ayatollah Ruhollah Khomeini. During Saddam Hussein's rule, the movement used Iraq as a base for operations against Iran's government. Thousands of its members remain in Iraq, and both the US and Iraq consider the Khalq a terrorist organization. In addition to the Khalq members, Mshawi said five others detained Saturday belong to a Shiite cult group. He did not elaborate or give details about the group. The remaining six suspects were "wanted men," Mshawi said. Meanwhile north of Baghdad, Iraqi police said a local al-Qaida in Iraq leader was killed in his home, and 12 decomposed bodies were discovered in a mass grave. Abu Omar al-Dori resisted police for about an hour before he was killed around 4 a.m. in his house in downtown Samarra, a police officer said. He spoke on condition of anonymity because he was not authorized to speak to media. Samarra is a mostly Sunni town about 95 kilometers north of the Iraqi capital. According to Iraqi police, al-Dori had been assigned to lead al-Qaida in Iraq operations in Samarra just one week ago. It was unclear whether his predecessor was killed or captured. Farther east near Baqouba, a joint patrol of Iraqi police and soldiers found a mass grave with 12 bodies, including three of women, according to police and morgue officials. The bodies were found in the al-Ehaimer area on the outskirts of Baqouba, 60 kilometers northeast of the capital. The US military had no immediate comment on either incident. Meanwhile, Iraqi President Jalal Talabani traveled Saturday to Najaf, another Shiite holy city south of Baghdad, to meeting with Grand Ayatollah Ali al-Sistani, Iraq's most prominent cleric. It was unclear whether the meeting was scheduled in light of the recent Shiite arrests. Talabani was expected to hold a news conference later Saturday. Saturday's arrests south of Baghdad came two days after al-Sadr's office issued a statement warning Mahdi Army fighters to stick with his cease-fire order, after US and Iraqi raids Thursday in Baghdad's Sadr City - the main Shiite district and Mahdi Army bastion. In the statement, al-Sadr threatened to expel militiamen from his group who break the cease-fire. The order is credited with helping tamp down violence dramatically in Baghdad, along with the arrival of about 30,000 US reinforcements last summer. Fifteen suspected militants were detained in Thursday's sweeps through Sadr City, and one person was killed. The US similarly said it was targeting "criminal elements" responsible for attacks with mortars and powerful roadside bombs that the Pentagon links to Iranian aid.