A suicide car bomber attacked a police station on the outskirts of one of Iraq's major oil hubs on Saturday, killing at least four policemen and injuring eight, police said. The attack came a day after the bombing of a key oil pipeline just east of the northern oil city of Beiji. Saturday's explosion occurred at 9:30 a.m. at a police station about five kilometers north of the city center, police said. Beiji - which lies 250 kilometers north of Baghdad - houses northern Iraq's largest oil refinery, and serves as a key transfer point for crude oil being exported out of Iraq. Attacks on the Kirkuk-Beiji pipeline cluster have been infrequent in recent months, with some 300 US soldiers and more than 2,000 Iraqi troops guarding the zone.