Militants attacked Iraqi police with guns and explosives and lobbed a mortar round at a security headquarters on Wednesday, killing four people and wounding dozens in the latest assault on security forces.
Insurgents deployed roadside bombs, a car bomb and a hand grenade as they launched at least eight attacks on police in the Iraqi capital, Baghdad, and northern Mosul, an al Qaeda stronghold, where three people were killed.
Militants are testing Iraq's army, police and fragile governing coalition as US troops prepare to withdraw by a year-end deadline, more than eight years after the invasion that ousted Sunni dictator Saddam Hussein.
"I think political instability is the main reason why violence has escalated recently," said Abdul Rahim al-Shimmari, head of the Nineveh provincial council's security committee.
"Some political parties own armed militias and huge funding and they use the worsening security situation as a pretext to create the legitimacy for keeping U.S. forces in Iraq."
Violence has fallen sharply in recent years after the sectarian slaughter of 2006-07 but a tenacious Sunni Islamist insurgency linked to al Qaeda and rival Shi'ite militias still carry out scores of bombings and other attacks every month.
Iraqi government officials and security forces are under attack as the remaining American troops, about 47,000, prepare to leave the OPEC oil producer by the end of December.
Wednesday's spate of attacks killed at least one policeman and three civilians, and more than a dozen police were among 32 people wounded.
The mortar shell fired at the Nineveh security centre, a headquarters used by the army and police, missed the target and hit a house, killing one person and wounding another in southern Mosul, 390 km (240 miles) north of Baghdad, police said.
Attackers threw a grenade at a police patrol in Mosul, wounding four people, and killed an officer at a security checkpoint, while a roadside bomb near a patrol killed a bystander and wounded two people, including a policeman.
In the capital, a local police chief and five officers were wounded when two roadside bombs struck their convoy in the western Amiriya district, an interior ministry source said.
In the western Ghazaliya district, a parked car bomb exploded near a police patrol, the source said, killing a bystander and wounding nine people, including three police.
Bombs targeting police wounded three officers and five civilians in the Zayouna and Jadiriya areas, the source said.
A senior Iraqi security official who asked not to be named said the recent escalation of attacks was expected.
"I think our security forces are still unfit to have complete control of the security situation," the official said. "More combat training and more expertise are needed."
Near Iraq's southern oil hub, Basra, a US military convoy was struck by a bomb blast, police said. A US military official said no one was hurt.