At least 13 Iraqi soldiers have been killed in an ambush in the northern city of Mosul, police say.
Gunmen opened fire at a checkpoint in the Abdaiyah area of Mosul, also wounding four soldiers, police said.
In other violence, US forces killed three Iraqi policemen in a "friendly fire" incident in a raid on suspected insurgents in Ramadi, west of Baghdad.
The military said US forces had come under small arms fire and returned fire. Seven insurgents were arrested.
Army Major Qassim Abdullah in Mosul told the AFP news agency: "A large group of terrorists drove up in five cars and opened fire immediately on the al-Hadhar checkpoint, before making off."
Also in Mosul, gunmen reportedly killed a university professor near his house.
In another town north of Baghdad, Hawija, gunmen reportedly killed the imam of a Sunni mosque and three bodies were found near the town of people shot and tortured.
Millions of Iraqis have fled the instability that is gripping the country, and Monday's violence came as a human rights group appealed for urgent international action to help refugees.
Amnesty International says neighbouring Jordan and Syria are running out of resources and struggling to care for approximately two million Iraqis.
In March, the United Nations refugee agency said there had been an abject refusal around the world to recognise the humanitarian impact of the invasion of Iraq.
It is estimated that about 50,000 Iraqis a month are now leaving their country.