The U.S. military says four helicopter engines worth $13 million are missing in the Afghanistan-Pakistan region.

U.S. spokeswoman Lt. Col. Rumi Nielson-Green says the helicopters were being shipped overland from the U.S. base in Bagram to a seaport for shipment back to Fort Bragg, North Carolina. She says the parts went missing sometime before the 101st Airborne's arrival in Afghanistan in April.

Nielson-Green says the parts were being shipped by a Pakistani trucking company, but it is not clear where they disappeared.

The U.S. is not disclosing what kind of engines they were.