Baghdad, Iraq (CNN) -- At least 30 Iraqi children riding in a bus were among the 160 people killed in Sunday's twin car bombings in Baghdad, the Interior Ministry said Monday.

At least 540 people were wounded in Sunday's attacks, the deadliest in the capital in more than two years, the ministry said.

One of the bombs exploded outside Baghdad's governorate building, the other outside the Justice Ministry. The bombs detonated in quick succession about 10:30 a.m., officials said.

The children were packed in a mini bus that was outside the Justice Ministry building, a ministry official said.

The vehicle carrying the explosives that detonated outside the ministry building was a stolen white pick-up from Falluja, Baghdad Gov. Salah Abdul Razzaq told CNN during his inspection of the bomb site. Images from the time of the attack showed the truck, linked to the Department of Water, pull up to the side of the building and blow up, he said.

Plumes of smoke billowed from the sites as victims fled, some with blood streaming down their faces. The streets were strewn with debris, including charred cars and chunks of concrete. Some government buildings and others in the area were heavily damaged.

Among the wounded were three American security contractors, the U.S. Embassy said, declining to provide further details. The area struck is close to the heavily guarded "Green Zone," which houses the embassy.

The blasts sparked questions about Iraq's security and national elections planned for January.

Iraqi Prime Minister Nuri al-Maliki, who visited the scene shortly after the explosions, said holding the elections as scheduled would send a strong message to the attackers.

"The cowardly attack ... should not affect the determination of the Iraqi people from continuing their battle against the deposed regime and the gangs of criminal Baath party, and the terrorist al Qaeda organization,'" al-Maliki said in a written statement.

U.S. President Barack Obama called the attacks an attempt to derail progress in Iraq, and pledged to work closely with the country as it prepares for elections. Obama spoke with the prime minister and President Jalal Talabani to express his condolences and reiterate U.S. support.

In August, more than 100 people were killed in a series of bombings that led to tightened security in Baghdad. Blast walls were installed across the city and checkpoints were added.

Two years earlier, three truck bombings killed hundreds in Qahtaniya, in northern Iraq. Sunday's attacks were the deadliest on Iraqi civilians since those blasts in August 2007.

A day before Sunday's explosions, the U.S. ambassador to the United Nations, Susan Rice, visited Iraq for the first time. During her trip, she made a condolence stop at the Foreign Ministry, one of six sites attacked in August.

Iraqis are supposed to go to the polls January 16, but parliament has not passed key election legislation, putting the balloting in limbo.

The president, prime minister and other top officials are scheduled to meet Monday to discuss the elections law and security concerns.

CNN's Jomana Karadsheh, Mohammed Jamjoom, and Mohammed Tawfeeq contributed to this report.