BAGHDAD, Iraq (CNN) -- Insurgents in Iraq targeted Shiite Muslims on Sunday -- the second day of the Eid al-Fitr festival -- in separate attacks that left at least 24 dead, Iraqi officials said.
The deadliest attack happened in Samarra, north of Baghdad, where a car bomb detonated near a mosque in the city's center. The explosion was followed by clashes between gunmen and Iraqi security forces, according to Sammara police.
At least 18 were killed -- 10 civilians and eight security officers -- and 37 were wounded in the blast and gunfight in Samarra, police said.
Insurgents on Sunday also bombed a minibus in Baghdad as it carried pilgrims to one of Shiite Islam's holiest shrines, killing at least six and wounding nine others, including women and children, Iraqi officials said.
It was unclear where the bomb was placed. An Iraqi Interior Ministry official said the device was planted on the bus heading to the Imam Musa al-Kadhim shrine in Baghdad's Kadhimiya district. But the Ministry of Defense said the explosion came from a bomb in a car parked near the bus.
The bombing happened near Aden Square, where Iraqi security forces dismantled another parked car bomb earlier Sunday.
Iraqi police also found and defused an explosive device in a van parked near an air base in central Baghdad.
The deadly bombing in Samarra was the second this weekend in the Shiite holy city, about 75 miles (120 kilometers) north of Baghdad in Salaheddin province.
The two attacks happened less than a mile (about a kilometer) apart.
At least four Iraqi police commandos were killed Saturday when a suicide car bomber detonated outside a building where the police were stationed, police said.
Nine other commandos were wounded in the blast. Insurgent fire then sparked a gunbattle with police.
Sunday is the second day of Eid al-Fitr for Shiites, the feast marking the end of fasting during the holy month of Ramadan.
Meanwhile, one of Iraq's top Shiite Muslim leaders visited the Sunni city of Ramadi on Sunday for meetings with tribal leaders who have allied themselves with American forces.
Ammar al-Hakim, the son of the leader of the Supreme Iraqi Islamic Council, visited what was once the heart of the insurgency against American forces for talks with provincial officials and members of the Anbar Awakening Council. He met with the group's leader, Sheik Ahmed Abu Reesha, during Sunday's visit.
U.S. leaders have touted the success of such groups, led by Sunni Arab sheiks, as a sign of "bottom-up" progress in the 4-year-old war even as nationwide efforts for a political settlement have faltered.
But U.S. support for the groups -- which include former insurgents -- has raised alarm among Shiite leaders. Speaking during a Saturday sermon marking the end of the Islamic holy month of Ramadan, al-Hakim called for caution in arming the Sunni alliances, saying their members "should be vetted and their loyalties checked."
Al-Hakim was accompanied by a high-level delegation from his party, the largest bloc in Iraq's parliament and the cornerstone of the ruling Shiite alliance.
Ammar al-Hakim is the son of SIIC leader Abdul Aziz al-Hakim, who returned to Baghdad on Wednesday after completing a phase of cancer treatment in Iran.
Also on Sunday, a U.S. soldier with the Army's Task Force Lightning died in a "a non-combat related incident" in Nineveh province, the U.S. military said.
"The incident is under investigation," the military said in a news release.
The latest fatality brings the U.S. military death toll in Iraq to 3,821. Of those, 705 have been in non-hostile situations. Seven Department of Defense civilians have also been killed.
Also, a correspondent for The Washington Post was fatally shot while on assignment in Baghdad, the newspaper said.
Salih Saif Aldin, 32, an Iraqi who sometimes wrote under the name Salih Dehema for security reasons, was shot in the forehead while reporting on violence in the Baghdad neighborhood of Sadiyah, the paper reported.
He was the first reporter for the Post to be killed during the Iraq war, the newspaper said. At least 118 other journalists, most of them Iraqis, have also died covering the war, according to the Committee to Protect Journalists. E-mail to a friend 
CNN's Jomana Karadsheh contributed to this report.
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