Afghan officials say Taliban militants have killed 15 local guards working for a private U.S. security firm in Afghanistan.

Officials say the Afghan guards were escorting a supply convoy in the western province of Farah when they were ambushed Tuesday. At least five other guards were wounded.

The convoy was on its way to a military outpost in Helmand province, the volatile heartland of Afghanistan's poppy-producing region.

Also Tuesday, a statement attributed to Taliban leader Mullah Omar vowed his group will continue to attack coalition troops during the Afghan winter.

The one-eyed fugitive cleric said Taliban fighters will keep pressure on Musa Qala, the town recently recaptured by coalition forces after 10 months under Taliban control.

Separately in Farah province, Afghan officials say two Taliban militants were killed Monday during clashes with police in the Khaki Safed district.

And U.S.-led coalition forces said they have killed several militants in an airstrike targeting a Taliban commander in Helmand province.

Violence fueled by a Taliban insurgency has killed more than 6,000 people in Afghanistan this year.

Some information for this report was provided by AFP and Reuters.