BAGHDAD, (AP) – U.S. forces fired an artillery barrage in southern Baghdad Sunday morning, rocking the capital with loud explosions, while the death toll from a suicide car bomb attack in the Shiite holy city of Karbala rose to 68.

The blasts in Baghdad came a day after the U.S. military announced the deaths of nine American troops, including four killed in separate roadside bombings south of Baghdad and five in fighting in Anbar province, a Sunni insurgent stronghold west of the capital.

The size and the pattern of the explosions, which began after 9 a.m. and lasted for at least 15 minutes, suggested they were directed at Sunni militant neighborhoods along the city’s southern rim. Such blasts have been heard in the evenings but are rare at that time of day.

In a brief statement to The Associated Press, the U.S. military said it fired the artillery from a forward operating base near Iraq’s Rasheed military base southeast of Baghdad, but provided no other details.

Iraqis in the southern region of the city said American and Iraqi forces had stepped up their operations in the Dora area of southern Baghdad starting Saturday night.

Meanwhile, Iranian President Mahmoud Ahmadinejad confirmed Sunday that his government will attend a major regional conference on Iraq set for this week in Egypt, the Iraqi prime minister’s office said.

A statement from Prime Minister Nouri al-Maliki’s office said Ahmadinejad telephoned the Iraqi leader and told him Tehran would participate in the meeting Thursday and Friday in the resort of Sharm El-Sheik.

The call was made as top Iranian envoy Ali Larijani flew to Baghdad for talks with Iraqi leaders.

In Tehran, Foreign Ministry spokesman Mohammad Ali Hosseini said Larijani would discuss the conference with Iraqi government officials as Iran has “some questions and ambiguities about the agenda.”

Authorities in northern Iraq imposed an indefinite curfew in the Sunni stronghold of Samarra after leaflets signed by rival insurgent groups threatened policemen if they did not quit their jobs and promised to target any oil company that wants to explore in the area.

The warnings to the policemen were signed by al-Qaeda in Iraq and threatened to destroy their houses if they didn’t comply.

Leaflets signed by a separate insurgent umbrella group calling itself the Mujahedeen of Samarra warned against oil exploration in the area and were posted on the walls of mosques in central Samarra, 60 miles north of Baghdad.

The blast in Karbala, 50 miles south of Baghdad, took place about 7 p.m. Saturday in a crowded commercial area about 200 yards from the shrines of Imam Abbas and Imam Hussein, major Shiite saints. The shrines, some of the country’s most sacred, were not damaged, police said.

Police first thought the explosion was caused by a parked car bomb, but Ghalib al-Daami, a Karbala provincial council member, said Sunday it was a suicide car bomber in the busy commercial center.

Residents on Sunday gathered around the large crater left in the road by the bomb, and pools of water left by firefighters fighting the blaze were still tinged red with blood.

Salim Kazim, the spokesman for Karbala health directorate, said the casualty figures had risen to 68 dead after some of the wounded died and more bodies were found on the roofs of buildings or in the rubble, and 178 were wounded.

As police maintained a vehicle ban in the city, 50 miles south of Baghdad, people began digging through the rubble of damaged shops.

“The explosion was so powerful that it threw me up into the air,” said Haidar Ismail, one of the many patients lying in overcrowded rooms at Imam Hussein Hospital with bandages covering their wounds and burns.

Saturday’s attack was the second car bomb to strike the city’s central area in two weeks. On April 14, 47 people were killed and 224 were wounded in a car bombing in the same area.

Ghalib al-Daami, a provincial council member who oversees security matters, said the bomber detonated his payload about 200 yards from the Imam Abbas shrine, which with the others draws thousands of Shiite pilgrims from Iran and other countries. That suggested the attack was aimed at killing as many Shiite worshippers as possible.

“I did not expect this explosion because I thought the place was well protected by the police,” said Qassim Hassan, a clothing merchant who was injured. “I demand a trial for the people in charge of the security in Karbala.”

Speaking from a hospital bed, Hassan said his brother and a cousin were still missing. “I regret that I voted for those traitors who only care about their posts, not the people who voted for them,” he said.

The U.S. military has warned that such bombings were intended to provoke retaliatory violence by Shiite militias, whose members have largely complied with political pressure to avoid confrontations with Americans during the U.S. troop buildup.

In other developments Sunday:

• Two roadside bombs exploded in separate areas in a predominantly Shiite area in southeastern Baghdad, killing three civilians and wounding nine, police said.

• Gunmen seriously wounded Amal al-Moudares, one of Iraq’s best known radio and television journalists, in an attack near her home in Baghdad, police said.

• The Islamic State of Iraq, an umbrella group of Sunni militants that includes al-Qaeda in Iraq, claimed responsibility for a suicide truck bombing Friday in the western city of Hit, saying it was targeting the police chief. The attack killed nine Iraqi security forces and six civilians, although police chief Hamid Ibrahim al-Numrawi and his family were unharmed.

• In a statement posted on a militant web site, al-Qaeda in Iraq claimed responsibility for a suicide car bomb Thursday that killed 10 Iraqi soldiers at a checkpoint in Khalis, a longtime flashpoint city 50 miles northeast of Baghdad.