A suicide car bomb exploded at a checkpoint manned by US-backed Sunni tribesmen in northern Iraq on Sunday, wounding four of the men, police said. The attack took place around 11 a.m. near Hawija, about 50 kilometers (30 miles) southwest of Kirkuk. The area, a longtime Sunni militant stronghold, has seen an increase in violence as militants flee northward away from U.S.-Iraqi offensives around Baghdad and its surrounding belts. But back in November, Hawija hosted what the US military called the largest Sunni volunteer effort since the Iraq war began. Nearly 6,000 Sunni residents joined forces with the Americans - manning checkpoints across the area - to oust al-Qaida-inspired militants from their hometowns. Four of those security volunteers were wounded in Sunday's car bombing, said Brig. Sarhad Qadir of the Kiruk police department. One of the men suffered severe injuries, he said.