BAGHDAD - Islamic State militants attacked the outskirts of Iraq's northern oil refinery town of Baiji overnight with car bombs and clashed with the army and Shi'ite militias in the town's western districts, the local mayor and security sources said on Friday.The town of Baiji and its refinery - Iraq's largest - have been a battlefront for more than a year. The hardline Islamists seized the town in June 2014 as they swept through much of northern Iraq towards the capital Baghdad.Control of Baiji neighborhoods has changed hands many times during the conflict. Authorities said last month they had recaptured most of the town, but the radical jihadist group attacked central neighborhoods days later, forcing pro-government forces to pull back.At least three militia fighters were killed on Friday when Islamic State launched car bomb attacks against a makeshift headquarters in al-Rayash, about 18 km (5 miles) south of Baiji, a militia leader and a source in the Salahuddin Operations Command said.Mortars in an adjacent area killed two civilians, according to the source in the operations command, the Iraqi military's command center for the province of Salahuddin, where Baiji is situated.Baiji Mayor Mahmoud al-Jabouri said Islamic State used 12 car bombs and more than 200 fighters in the offensive."The attack started at 2 am," he said. "They came from the direction of Camp Speicher," a former US base outside the city of Tikrit.Jabouri said "tens" of security forces and militia fighters were killed or wounded in the violence, which also saw ongoing clashes in Baiji's western districts of Sikak and Tamim.