Shootings and bombings in Iraq killed at least 21 people Thursday, officials said, as the country struggles with rampant violence ahead of parliamentary elections at the end of the month.

The country is suffering from a protracted surge in bloodshed that has killed more than 2,500 people this year and sparked fears Iraq is slipping back into the all-out sectarian fighting of 2006-2007.

The unrest has been driven principally by complaints among the Sunni Arab minority of mistreatment by the Shiite-led government and security forces, and by the civil war in neighbouring Syria.

In Thursday's deadliest single incident, a car bomb exploded near a petrol station in the Ameen area of east Baghdad, killing at least seven people and wounding 35, officials said.

Another car bomb exploded in an area of shops in the capital's northern Shiite-majority Sadr City district, killing at least six people and wounding 18.

The attacks came after eight car bombs hit the capital the day before.

In a bid to cause maximum casualties, militants frequently target areas where crowds gather, such as shops, markets, mosques and cafes.

North of Baghdad, gunmen opened fire on a group of young Shiite men in Khales, north of Baghdad, and killed five of them, a police colonel and a doctor said.

Fundamentalist Sunni militants frequently target members of Iraq's Shiite majority, whom they consider apostates.

In a market in Baquba, also north of the capital, a gunman shot dead an army captain, while a roadside bomb killed a woman near the city.

And in the northern province of Kirkuk, a member of powerful jihadist group the Islamic State of Iraq and the Levant killed his father, apparently because he refused to leave a rival militant group, security sources said.

On April 30, Iraqis go to the polls for the first parliamentary election since US forces pulled out in 2011.

The election will be a major test for security forces, who were able to keep violence to a minimum during last year's provincial polls, but have subsequently failed to bring a year-long surge in unrest under control.

Violence has killed more than 290 people this month, according to AFP figures based on security and medical sources.