An Al-Qaeda-linked group claimed a brazen attack on a ministry and a bombing in south Iraq left 10 people dead Sunday as a study said at least 112,000 civilians were killed since the 2003 US-led invasion.

Ahead of the anniversary of the invasion that ousted Saddam Hussein, the latest violence raises fresh questions about the security forces' ability to prevent attacks such as the March 14 assault on the justice ministry claimed by the Islamic State of Iraq (ISI).

The attack involved a series of mid-day bombings in central Baghdad's Allawi neighbourhood, adjacent to the Green Zone which is home to several government buildings and the American and British embassies.

As the bombs went off, militants stormed the ministry complex, clashing with security forces.

Accounts differed as to the success of the attack, but one official said two insurgents managed to detonate suicide vests inside the ministry building.

Overall, 18 people were killed and more than 30 wounded, security and medical officials said. The ISI claimed to have killed 60 people, according to a statement translated by the SITE monitoring service on Sunday.

Militants struck again on Sunday, detonating a car bomb at a bus station near the outskirts of the southern port city of Basra, killing 10 people and wounding 16, the head of the provincial council's security committee said.

In addition to the casualties in the predominantly Shiite city, the blast turned several nearby vehicles into mangled wrecks.

A suicide bomber also wounded three police north of Baghdad, a police officer and a medical source said.

Britain-based Iraq Body Count (IBC), meanwhile, published a study which concluded that at least 112,000 civilians were killed in the 10 years since the invasion.

It said that, including combatants on all sides of the decade-long conflict as well as yet-undocumented fatalities, the figure could rise as high as 174,000.

"This conflict is not yet history," it said in its report, which put the number of civilian deaths since March 20, 2003 at between 112,017 and 122,438.

"It remains entrenched and pervasive, with a clear beginning but no foreseeable end, and very much a part of the present in Iraq."

IBC said that, over the years, Baghdad had been, and still is, the deadliest region, accounting for 48 percent of all deaths, while the conflict was bloodiest between 2006 and 2008.

It noted that violence remained high, with annual civilian deaths of between four and five thousand roughly equivalent to the total number of coalition forces killed from 2003 up to the US military withdrawal in December 2011, at 4,804.

In addition to Baghdad, the most violent regions were the northern and western provinces, dominated by the Sunni Arab minority which controlled Iraq during Saddam's rule but which has since been replaced in power by the Shiite majority.

The IBC report comes after a separate study published in British medical journal The Lancet said that at least 116,000 Iraqi civilians had died between 2003 and the 2011 American withdrawal.

The researchers behind the Lancet study pegged the total financial cost of the war for Washington at $810 billion (625 billion euros), warning that it could eventually reach $3 trillion.

Iraq's military and police are consistently described by Iraqi and American officials as capable of maintaining internal security, but are not yet fully able to protect the country's borders, airspace and maritime territory.

Attacks such as the March 14 assault still periodically occur, however, with militants striking other heavily defended targets such as police stations, prisons and government offices, in addition to carrying out waves of shootings and bombings targeting civilians and government officials nationwide.

In February, 220 people died in attacks in Iraq, according to an AFP tally based on reports from security and medical officials.