Disguised suicide bombers have killed seven people and wounded 12 others in an attack on a power plant north of Baghdad.

General Qassem al-Tamimi, head of a police unit in charge of protecting installations, said three attackers wearing military uniforms and armed with grenades entered the facility near the city of Samarra early on Saturday.

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A police officer, who spoke on condition of anonymity, told AFP news agency one of the bombers detonated his explosives belt while two others were shot dead by security reinforcements who rushed to the scene in Samarra, about 100km north of the capital.

He said seven people were killed and 12 wounded in the attack.

Police and medical sources from a nearby hospital told Reuters news agency four policemen and three workers were killed in the assault, putting the number of wounded people at 13.

ISIL claims responsibility

Islamic State of Iraq and the Levant (ISIL, also known as ISIS) claimed responsibility for the attack in a statement.

"I was on my night shift and suddenly heard shooting and blasts. A few minutes later, I saw one attacker wearing a military uniform and throwing grenades through the windows," said Raied Khalid, a worker who was wounded by shrapnel.

"At 2am we were woken up by shots being fired," Abdel Salam Ahmed, an employee who was hit by gunfire in the legs, told AFP from his hospital bed.

He recalled running with colleagues away from the shooting.

"We ran into one of them. Some of us hid while two others kept running towards the exit, shouting 'we are employees' but they shot them dead," said Ahmed.

Several tanker trucks were also damaged and the remains of one suicide bomber lay on the ground.

The police official said security reinforcements evacuated the employees.

In 2014, ISIL captured almost one-third of Iraqi territory in a lightning offensive. It now only holds two pockets of territory in the country.