A series of attacks north of Baghdad -- including multiple bombings targeting police -- killed 13 people on Monday, as Iraq grapples with its worst bloodshed since 2008.

Prime Minister Nuri al-Maliki has appealed to Washington for greater cooperation in fighting militancy as wide-ranging operations targeting insurgents and tightened security measures have done little to quell a months-long surge in violence.

Monday's violence concentrated on security forces for a second consecutive day, with suicide bombers hitting police sites in multiple cities in north Iraq.

In the deadliest attack, multiple bombings targeting a police station in the predominantly Sunni Arab town of Sharqat, in Salaheddin province, left four policemen dead and a dozen more wounded, according to police and medical sources.

An initial car bomb outside the station caused no casualties, but as police and emergency responders gathered at the scene of the blast, two suicide bombers detonated their explosives-rigged belts.

Another car bomb, this one set off by a suicide bomber, targeted a police academy in Salaheddin's capital Tikrit, just a day after the centre launched a recruitment drive.

One person was killed, and nine others were wounded by the explosion, officials said.

Two suicide bombers also targeted a police station in the disputed, ethnically-mixed northern province of Kirkuk, killing two policemen and wounding seven others.

One of the suicide bombers blew himself up at the entrance to the station, causing the casualties, officials said. The other had holed up inside, with security forces evacuating the building and sealing off the area, before blowing himself up without causing casualties.

The latest attacks came a day after another coordinated set of bombings against a police headquarters in the restive central city of Baquba killed three policemen.

Also on Monday, gunmen shot dead three civil servants and a policeman in separate shootings in the main northern city of Mosul and left another wounded, and explosions in two different towns just north of Baghdad killed two people, including a soldier.

Violence so far this year has left more than 5,400 people dead, the country's worst violence since 2008, when it was emerging from a brutal sectarian war in which tens of thousands were killed.

In addition to major security problems, the Iraqi government has failed to provide adequate basic services such as electricity and clean water, and corruption is widespread.

Political squabbling has paralysed the government, while parliament has passed almost no major legislation in years.