U.S. officials are investigating allegations that some Afghan Air Force officers have been using official aircraft to fly drugs and illegal weapons around the country.

A NATO spokesman says the investigation is still very preliminary.

A U.S. newspaper, The Wall Street Journal, says investigators are also looking into whether the alleged air force drug and weapons trafficking is related to a deadly attack last year. In the attack, an Afghan military pilot opened fire on NATO troops, killing eight U.S. soldiers and an American contractor. It was the deadliest such incident since the war began in 2001.

The Journal reports the attacker, an air force colonel, is accused of being involved in the illegal activities, while most of the victims were involved in an early phase of the investigation.

Afghanistan produces 90 percent of the world's opium, and the United Nations says as much as 15 percent of Afghanistan's Gross Domestic Product comes from the drug trade.