Gunmen have attacked a football game north of Baghdad, killing a policeman and a Sunni Muslim allied with the US against al-Qaeda, the US military says.

The attack near Duluyiah, 75km north of Baghdad, also wounded three others, including a nine-year-old and a second member of the local US-backed awakening council, the military said.

"To fire upon a soccer game where the people of Iraq are starting to enjoy normalcy and happiness is another example of the barbaric nature of the insurgents," Major Peggy Kageleiry, a US military spokeswoman, said in a statement.

Violence in Iraq is at its lowest level in four years, but frequent attacks continue throughout the country.

A roadside bomb exploded near the house of a police captain 65km west of Baghdad in Fallujah on Sunday, killing four policemen and wounding eight others, police said. The wounded included four policemen and four civilians.

All those caught in the blast were drawn to the location by an initial roadside bomb that exploded 10 minutes earlier, police said, speaking on condition of anonymity because they were not authorised to talk to the media.

Also on Sunday, a roadside bomb struck a truck near Baqouba, 60km northeast of Baghdad, police said. The blast injured the driver and his assistant, who later died of their wounds at a nearby hospital, said the police, speaking on condition of anonymity.

The decline in violence in Iraq has been driven by a variety of factors, including the 2007 US troop surge, the awakening council movement and Iraqi government crackdowns against various armed groups throughout the country.

The government has led operations against Shi'ite militias in the southern cities of Basra and Amarah and Baghdad's Sadr City. It also has launched offensives against Sunni extremists in northern city of Mosul.

Iraq's interior ministry spokesman Major General Abdul-Karim Khalaf said Sunday that the government would soon launch an operation in the eastern province of Diyala.

Last year, US troops regained control of Diyala's capital of Baqouba, which had been held by al-Qaeda in Iraq and other Sunni militants.

But local officials fear al-Qaeda militants are returning to the province to escape pressure from US and Iraqi forces in the Mosul area. Last month, Prime Minister Nouri al-Maliki singled out Diyala province as a possible next target for a military offensive.

"The Diyala operation will be the last surge," Khalaf told a press conference.