The policemen died after approaching an empty car parked on the main highway outside Khan Bani Sad to inspect it when it exploded, Iraqi army and medical sources said.

A civilian driving by also died, while two civilians and a policeman were wounded by the morning blast, 20km north of the capital.

The attack followed the killing Friday of a police chief in nearby Baladruz and the bombing by rebels of the landmark 9th century Malwiya spiraling minaret in Samarra.

Meanwhile, in a radical policy shift, Sunni clerics, some of them with back channels to the insurgency, issued a statement calling on their community to join the Iraqi security forces.

But amid the continuing violence, efforts by Iraq's Shia majority to bring the country's Sunni minority into the government had still not born fruit.

The Shia United Iraqi Alliance (UIA), which controls 146 seats in parliament, said it would veto Sunni MP Mishaan al-Juburi from taking becoming speaker and told Sunnis who did not like the final choice to leave the parliament.

"We have agreed on the nominees, and the candidate for the speakership has been endorsed by a majority of Sunnis," Jawad al-Maliki, a senior member of the UIA, told AFP.