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NEAR KIRKUK, Iraq — Armed militants, believed to be wearing suicide vests, attacked the Bai Hassan oil field near the northern Iraqi city of Kirkuk on Sunday and have taken four workers hostage, according to a senior security source in the city.

One attacker remains in the facility, while two were killed by security forces and another detonated himself when security forces responded. The number of casualties is not known.

The ISIS-affiliated Amaq news agency has released a statement claiming responsibility for the attack.