At least 11 police officers and members of the Afghanistan Interior Ministry’s anti-narcotics department were killed and four of them were injured by an airstrike in the southern province of Helmand Sunday night, the Associated Press reported. According to one Afghan official, NATO carried out the airstrike, but Army Col. Brian Tribus, a representative of U.S. forces in Afghanistan, told AP there were no American or NATO airstrikes in Helmand Sunday.

However, NATO did conduct an airstrike in Kandahar Sunday to “protect the force,” Tribus said. Meanwhile, Nabi Jan Mullahkhail, the Helmand provincial police chief, confirmed the airstrike in his province, but did not say who carried it out. Afghanistan’s own air force has the capability to conduct airstrikes, but both it and the nation's defense ministry declined to comment about the incident, Agence France-Presse reported.

The airstrike happened outside Lashkar Gah, the Helmand provincial capital. It occurred during an operation intended to arrest drug smugglers, according to an Afghan official. The area attacked is where the government’s anti-drug police forces have frequently set up ambushes to intercept drug smugglers, Al Jazeera reported. Afghanistan is the world’s largest producer of opium, which is refined to make heroin.

U.S.-led NATO forces concluded their combat mission for the most part in Afghanistan last December. However, there are still 13,000 U.S. military personnel in the country engaged in counterterrorism operations and training.

And in recent months, there has been an increase of airstrikes carried out by NATO in Afghanistan. Coalition forces conducted 106 of them in June, a significant increase over the 41 conducted in May, according to NATO statistics, AFP reported. In July, officials said at least 14 Afghan soldiers were killed by a U.S. airstrike in a Taliban-infested province south of Kabul.