A wave of more than a dozen car bombings hit central and southern Iraq during morning rush hour on Monday, killing at least 51 people, officials said, in the latest co-ordinated attack by insurgents.

The blasts, which wounded scores more, are the latest in a surge in attacks over recent months, reviving fears of a return to the widespread sectarian bloodshed that took the country to the brink of civil war after the 2003 US-led invasion.

Suicide bomb attacks, car bombings and other violence have killed more than 3,000 people since April, including more than 500 since the start of July, according to an Associated Press count.

There was no immediate claim of responsibility for Monday's attacks, but they bore the hallmarks of al-Qaida's Iraqi arm. The group, known as the Islamic State of Iraq, frequently sets off such co-ordinated blasts in an effort to break Iraqis' confidence in the Shia-led government.

Police said a total of 12 parked car bombs targeted markets and car parks in predominantly Shia districts of Baghdad within an hour of each other. They said the deadliest were in the eastern Shia neighbourhood of Sadr City, where two separate explosions killed nine civilians and wounded 33 others.

Ambulances rushed to the scene where rescuers and police were removing the charred and twisted remains of the car bombs from the bloodstained pavement. The force of the two explosions damaged nearby houses and shops.

Ali Khalil, 36, a taxi driver, said he was passing nearby when the first bomb exploded. "I heard a thunderous explosion that shook my car and broke the rear window," he said. "I immediately pulled over and didn't know what to do while seeing people running or lying on the ground."

He said he brought two of the wounded to a nearby hospital before heading back to his home to stay indoors for the rest of the day. Like many Iraqis he blamed political infighting and incapable security forces for the deteriorating security situation.

After the explosions, the acting UN envoy to Iraq, György Busztin, expressed concern over "the heightened level of violence which carries the danger that the country falls back into sectarian strife".

He said: "Iraq is bleeding from random violence, which sadly reached record heights during the holy month of Ramadan." He called for immediate and decisive action to stop the bloodshed.

Ten other bombs around Baghdad, mostly in Shia-dominated suburbs, as well as in the town of Mahmoudiya, 20 miles to the south, killed 31 people.

The bombings also extended to Iraq's majority-Shia south. Back-to-back explosions of two parked car bombs in an outdoor market and near a gathering of construction workers killed seven civilians and wounded 35 in the city of Kut.

In the oil-rich city of Basra, four people were killed and five wounded when a parked car bomb ripped through a market. Health officials confirmed the casualty figures.

The violence surged after a crackdown by security forces on a Sunni protest camp in the northern town of Hawija in April that killed 44 civilians and a member of the security forces, according to UN estimates.

The bloodshed is linked to rising sectarian divisions between Sunnis and Shias as well as friction between Arabs and Kurds, dampening hopes for a return to normality nearly two years after US forces withdrew from the country.