BAQUBA, Iraq (Reuters) - Attackers have slit the throats of a mother and three sisters of an Iraqi election commission employee in their home, security and medical sources said on Monday.

The employee himself, from the Turkmen minority in the town of Hamrin in ethnically mixed Diyala Province, was not at home at the time and was unharmed, the sources said. No group has claimed responsibility for the killings late on Sunday.

Islamic State threatened to attack Iraq’s May parliamentary election and anyone who assisted in it. At least one candidate was killed before the vote but the group did not claim responsibility for his killing.

The Sunni Islamist militants have lost the one-third of Iraq’s territory they once controlled, but still operate from pockets along the border with Syria and other areas including the Hamrin mountains.

Their ambushes, assassinations and bombings have increased in recent weeks, especially in Diyala.

A Diyala security source said Iraqi security forces had launched an operation in the north of the province against Islamic State militants.

Iraq’s parliamentary election on May 12 was marred by low turnout and allegations of fraud.

A manual recount of some votes is to take place, after allegations that electronic vote-counting equipment was misused. Parliament has replaced the leaders of the election commission with judges.