A coordinated string of bombings and a brazen assault on a ministry near Baghdad's heavily fortified Green Zone killed 18 people on Thursday, in the Iraqi capital's deadliest violence this month.

The attacks, which drew condemnation from the US, UN and Iraq's parliament speaker, come just days before the 10th anniversary of the US-led invasion of Iraq, and with barely a month to go before the country holds its first elections in three years.

No group immediately claimed responsibility for the violence, but Sunni militants including those linked to Al-Qaeda often target government officials and offices in a bid to destabilise Iraq.

At least three bombs exploded in Allawi neighbourhood, near the foreign and culture ministries and offices of the communications ministry, at about 1:30 pm (1030 GMT), officials and witnesses said.

At around the same time, militants staged an apparently unsuccessful assault on the nearby justice ministry.

Overall, 18 people were killed and at least 30 wounded in the attacks, an interior ministry official and a medical source said.

All of the buildings lie near the Green Zone complex in central Baghdad, home to parliament, the prime minister's office and the American and British embassies.

"Some terrorists tried to infiltrate the justice ministry," said Sabah Noori, spokesman for Iraq's Counter-Terrorism Service.

"The bombs... were coordinated with them (militants) trying to get into the ministry."

Gunfire was heard after the blasts, and smoke could be seen rising above the neighbourhood, witnesses said.

Security forces sought to bar the press from taking photos or video, according to an AFP journalist.

Accounts differed on how successful the militants were in their assault on the ministry.

An official in Baghdad's security command centre said three fighters were killed inside the justice ministry building, but ministry spokesman Haidar al-Saadi said clashes had occurred only outside.

An interior ministry official, however, said two fighters were killed in clashes while two others were suicide bombers who blew themselves up, one of whom did so near the justice minister's office.

Saadi said no ministry employees were hurt, and Iraqiya state television reported that security forces evacuated all employees from the building.

A justice ministry employee told AFP that staff escaped from the building via a rear entrance, and reported clashes between militants and security forces in which fighters used hand grenades.

Though such coordinated assaults are rare, Iraq has been struck before by them in previous years -- several ministries, police stations, prisons and provincial council offices have been hit by a combination of bombings followed by assaults in which militants tried to wrest control of official buildings.

Also on Thursday, a bombing targeted a candidate in Iraq's upcoming provincial elections, after another was kidnapped along with his father and other relatives north of Baghdad the night before.

A magnetic "sticky bomb" exploded on a car carrying Khaled Hussein al-Daraji, a candidate in Salaheddin province, killing his driver and wounding three nearby workers, although Daraji himself escaped unharmed.

And a tribal sheikh, Qais Abdul Karim al-Janabi, was kidnapped along with his son, Salaheddin provincial elections candidate Abdul Karim, and five other relatives in Siniyah, north of the capital.

Provincial elections are to be held on April 20, Iraq's first vote since March 2010 parliamentary polls.

Gunmen also wounded a soldier in the north Iraq city of Mosul on Thursday, while police found the body of a man who had been shot south of the city, police and a doctor said.

Violence has decreased from its peak in 2006 and 2007 when sectarian bloodshed raged between Sunni and Shiite Arabs.

But 10 years after the US-led invasion, attacks remain common, killing 220 people last month, according to an AFP tally based on security and medical sources.