U.S.-led coalition forces mistakenly killed five Afghan police in a clash in a southern province, the government said Friday.

The police were manning a checkpoint in Gereshk district, Helmand province when the clash broke out Thursday evening, said Interior Ministry spokesman Zemeri Bashary.

"Coalition forces mistakenly opened fire on police," Bashary said. "Unfortunately five policemen were killed."

A high-ranking Afghan delegation was sent to the area to investigate, Bashary said. He could not provide further details of how the friendly fire broke out.

U.S. military officials were not immediately available to comment on the matter.

The clash came as NATO-led forces pressed on with their largest operation yet in Afghanistan, trying to secure a region of Helmand province — a stronghold for resurgent Taliban militants.

Lt. Col. Angela Billings, a spokeswoman for NATO's International Security Assistance Force in Kabul, said their troops were not involved in the clash.

In all, about 47,000 Western forces are currently deployed in Afghanistan. NATO leads 36,000 troops, and the U.S.-led coalition maintains a 11,000-strong force engaged in independent anti-terror activities throughout the country.

Helmand is the center of the Taliban-led insurgency, and also the hub for the country's world-leading opium and heroin trade that is believed to help fund the Taliban but also profits Afghan officials.