A wave of car bombings in mainly Shia areas of Baghdad has killed at least 28 people, officials have said, the latest in a surge in violence that has been the most serious challenge to the government's efforts to achieve stability across Iraq.

The attacks on Tuesday came as people were celebrating the birthday of Imam Ali, the cousin and son-in-law of the Prophet Muhammad. No group immediately claimed responsibility for the attacks.

Police officials said a car bomb went off in the Baghdad neighbourhood of Sadr City, killing four people and wounding six in the morning hours.

Another car bomb elsewhere in Sadr City went off near a cluster of homes, killing two people and wounding seven.

Television footage from the AP news agency of one of the attacks showed thick smoke rising from the blast area where several cars were on fire.

A short while later, a car bomb exploded in a commercial street in Baghdad's eastern district of Jamila, killing three people and wounding 10.

Haithem Kadhum, owner of a juice shop in Jamila who was wounded in the attack, said he was in his store when he heard a big explosion.

He was told the blast was in his home neighbourhood of Sadr City, so rushed to his car to go check on his family.

But as he was driving through Jamila, another explosion went off, this one near him. The flying shrapnel wounded him in the shoulder.

"I was wounded in my right shoulder. I went out of the car and I saw dead and wounded people on the ground. Everybody was in panic,'' said Kadhum, after receiving treatment in a nearby hospital.

Commercial areas hit

Police said a fourth car bomb went off near a traffic police office elsewhere in the east of the capital, killing three people, including a traffic policeman. Seven people were wounded in the attack.

In central Baghdad, a car explosion killed two people and wounded eight, said police. Another car bomb went off in a commercial street in the city's Shia eastern district of Ur, killing five people and wounding 11.

Elsewhere in Baghdad, a car bomb exploded near an outdoor market in the eastern Shia suburb of Maamil, killing three people and wounding 14.

In the afternoon, a car bomb went off near a market in Baghdad's southern Dora district, killing five people and wounding 12 others.

According to the United Nations, 8,868 people were killed in Iraq last year, the country's highest death toll since the peak of sectarian bloodletting in 2007 and 2008.

Tuesday's violence was the deadliest since last month's election, from which votes are still being counted.

Preliminary results from the April 30 parliamentary election are expected within the coming days. Prime Minister Nouri

al-Maliki, a Shia Muslim, is seeking a third term despite worsening violence that critics blame in no small part on his policies towards the country's Sunnis.