It was not immediately clear if the Saturday night attack near Khanaqin, which also wounded some 20 others, was specifically targeting the Kurdish troops or a case of mistaken identity.
Parts of Khanaqin, located 150 kilometres (95 miles) northeast of the Iraqi capital, were held by militants while others were controlled by Kurdish forces as of Saturday, officers said.
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A major offensive by militants that overran swathes of territory has allowed Iraqi Kurds to begin realising long-held territorial dreams, moving their forces into areas that the federal government has long opposed them adding to their autonomous region.
The Kurds have at the same time joined the fight against the militants, who have overrun all of one province and chunks of three more since they launched their offensive late on Monday.
The federal government opposes the Kurdish incorporation of new territory, but with its forces only just starting to push back against the militants, it seems unlikely that Baghdad would seek to open a second front against the Kurds now.