The workers were taken on Friday when the IS militants attacked the oil facility of Khubbaz, some 25 km southwest of Kirkuk, which itself is located about 250 km north of Iraq's capital Baghdad, the source told.

During the attack, the IS militants stormed the facility, forcing the Kurdish security force, known as Peshmerga, to withdraw about three km away from the facility and then left the workers behind.

About two hours later, the Peshmerga made a counter-attack and retook control of the facility, but the 15 workers, who are affiliated to the Iraqi North Oil Company, were not found, the source said.

The IS militants carried out attacks at dawn Friday from three directions on the Peshmerga positions around Kirkuk, sparking heavy clashes with the Peshmerga forces.

On Saturday, sporadic clashes with heavy machine guns and mortar rounds continued in the area near the oil facility, while the Peshmerga, backed by the US-led coalition, managed to free seven villages in the area from the IS militants, the source added.

The security situation in Iraq began to drastically deteriorate June 10, 2014, when bloody clashes broke out between the Iraqi security forces and the IS group which took control of the country's northern province of Nineveh and later seized swathes of territories after Iraqi security forces abandoned their posts in other predominantly Sunni provinces.

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