Islamic State militants have set fire to oil wells northeast of the city of Tikrit to obstruct an assault by Shi’ite militiamen and Iraqi soldiers trying to drive them from the Sunni Muslim city and surrounding towns, a witness said. The witness and a military source said Islamic State fighters ignited the fire at the Ajil oil field to shield themselves from attack by Iraqi military helicopters. The offensive is the biggest Iraqi forces have yet mounted against IS, which has declared an Islamic caliphate on captured territory in Iraq and Syria and spread fear across the region by slaughtering Arab and Western hostages and killing or kidnapping members of religious minorities like Yazidis and Christians. Black smoke could be seen rising from the oil field since Wednesday afternoon, said the witness, who accompanied Iraqi militia and soldiers as they advanced on Tikrit from the east. […]