The crash occurred on the plane's approach to land at Ronald Reagan Airport near Washington DC.
The Federal Aviation Administration reported that the American Airlines Bombardier CRJ 700, operated by PSA Airlines, which was flying from Wichita, Kansas, crashed. It collided in mid-air with a UH-60 Black Hawk helicopter on approach to land.
There were 60 people and four crew members on board the aircraft, and the military helicopter was flown by three soldiers. The helicopter was on a training flight.
According to US media, at least 18 bodies were fished out of the river, although officially these figures have not been confirmed.
A rescue operation is underway.
Air traffic at the airport has been suspended.
The crash occurred on Wednesday around 9pm local time (4am on Thursday Lithuanian time).