An American Airlines jet and an Army helicopter collided over Washington, D.C., Wednesday night. The number of casualties is unclear and a search-and-rescue mission is ongoing.

A passenger jet has collided with a helicopter while landing at Ronald Reagan National Airport near Washington.

"Obviously, that will be fluid," John Potter, President and CEO of the Metropolitan Washington Airports Authority, said at a press conference early Thursday morning.Start the day smarter. Get all the news you need in your inbox each morning.