KABUL, Afghanistan — A recent move by the United States military to bar one of Afghanistan’s major airlines from contracts because of allegations of narcotics smuggling has caused a diplomatic firestorm, with Afghan officials angrily demanding proof and some American officials quietly criticizing the timing of the decision.

The blacklisting of the airline, Kam Air, is said to have started early this month and was first reported by The Wall Street Journal on Friday. The article quoted American military officials as saying the ban on American military contracts was based on information that the airline had carried “bulk quantities” of opium out of the country as well as within it.

Kam Air officials deny the accusation, and the Afghan government says it has no knowledge of such activities. President Hamid Karzai’s spokesman, Aimal Faizi, demanded that the American military hand over any evidence it had so that the government could act on the charges.

“We have to get to the bottom of this in order to take action,” Mr. Faizi said. “We need evidence and proofs in this regard. We cannot make inquiries based on a newspaper article.”