TIKRIT, Iraq (Reuters) - Suicide bombers struck a state-run power station north of Baghdad early on Saturday, killing seven people and forcing the facility to shut down in an attack claimed by Islamic State, police and army sources said. At least three gunmen wearing explosive vests attacked the power station around 3 a.m. local time, near the northern city of Samarra, about 100 km (60 miles) north of Baghdad. They used grenades to enter the facility.  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 injured by shrapnel. Security sources said the three gunmen briefly took control of the station, but police managed to regain control after three hours. Islamic State later claimed responsibility for the attack in a statement.   Four policemen and three workers were killed in the assault, in which 13 were wounded, police and medical sources from a nearby hospital said. One of the attackers, who was cornered by security forces, detonated his suicide vest near one the power generators, causing a fire.  The two other gunmen were killed, security sources said, either by blowing themselves up or in clashes with the security forces. Operations at the facility were expected to be suspended briefly, while repairs were under way, electricity officials said. 