BAGHDAD: A new wave of car bombs rocked commercial streets in the Iraqi capital on Tuesday, part of a series of attacks across the country that left 31 people and four attackers dead.

Meanwhile, Sunni leaders in Basra said unknown gunmen had shot dead 17 Sunnis in the Shia-dominated southern city over the past two weeks, following threats to retaliate against them for attacks on Shias in other parts of Iraq.

Car bomb attacks blamed on hard-line Sunnis aiming to undermine confidence in the Shia-led government, coming alongside revenge killings by Shias, are reminiscent of the cycle of violence that brought the country to the brink of civil war some years ago.

A surge of bloodshed is now in its fifth month, although overall death tolls are still lower than at the height of the conflict in 2004-2008.

Abdul-Karim al-Khazrachi, who leads the Sunni Endowment that oversees holy sites in Basra, said in a statement issued late Monday that the sect had decided to close down its mosques due to ''grave security deterioration and the continuation of the sectarian killings.''

Khazrachi told The Associated Press in a phone interview from Baghdad that the killings were preceded by threats, including letters that came with bullets in the envelopes, vowing revenge for insurgent attacks against Shias across Iraq.

The letters demanded that Sunnis leave the province. He said he didn't know the killers' identities.

The slain, he added, included clerics, worshippers and others.

The latest was a 70-year-old grocer who was shot dead by gunmen while standing in his store Monday night.

A police officer in the city confirmed the 17 killings, speaking anonymously as he was not authorised to talk to media.

Khazrachi said the mosque closure was intended to protect Sunnis but was ''also a message to all those in charge of the security to shoulder their responsibilities.''

Basra is Iraq's second-largest city.

It was controlled by militias for years before the US-backed Iraqi army gained control following a series of offensives in 2008.

In Baghdad, meanwhile, a wave of car bombs hit commercial districts.

One blast struck the northeastern suburb of Husseiniya in the late afternoon, killing five people and wounding 14 others.

Shortly before sunset, another bomb went off near a juice shop in central Baghdad, killing three and wounding 21.

Two more died in another car bomb explosion near a restaurant downtown, and in western Baghdad, two blasts killed six people and wounded 20 others.

A final two blasts hit a commercial street in the capital's southeast, killing seven and wounding 71.

In the western city of Fallujah, security forces foiled an assault on a police station, killing four attackers.

Two policemen were killed, officials said. And in the northern city of Mosul, gunmen stopped a mini-bus carrying soldiers to their base, killing six with shots in the head, police said.

Medical officials confirmed the casualty figures.

All spoke anonymously as they weren't authorised to release information.

Violence in Iraq surged in April after government troops moved against a camp of Sunni demonstrators in the town of Hawija north of Baghdad, triggering deadly clashes.

More than 4,000 people have been killed, including 804 just in August, according to United Nations figures. The monthly death tolls are the highest since 2008.