[Vance]

FORT DRUM, New York (CNN) -- A U.S. Army UH-60 Black Hawk helicopter on a training mission here crashed Tuesday, with an unknown number of people onboard, authorities said.

At least one soldier was seen walking around the crash site, said Fort Drum spokesman Lt. Col. Bryan Hilferty.

Rescue crews are en route.

The aircraft was flying with at least one other helicopter when radio contact was lost shortly before 2 p.m. , he said.

The Army said there was no smoke or debris seen at the time it dropped off the radar screen.

The Jefferson County Sheriff's Office said it was providing emergency support at the base and had dispatched four ambulances.

There was no word on how many troops were aboard, but the Black Hawk carries a crew of four and up to 11 passengers for a maximum capacity of 15.

Fort Drum, in northwestern New York, is home to the Army's 10th Mountain Division. However, it was not immediately clear if the troops were from that division or other Army units at the base.