The plane had been about to land at nearby Reagan National Airport after flying from Kansas.

American Airlines, whose subsidiary operated the flight, said "there were 60 passengers and four crew members on board the aircraft."

A US Army official said the helicopter involved was a Black Hawk model carrying three soldiers – their status currently unknown. They had been on a "training flight," a separate military spokesperson said in a statement.

Washington police said "there is no confirmed information on casualties at this time."

However, a massive search and rescue operation was in progress, with divers visible as they plunged into the snow-lined Potomac.

The Washington Post quoted unnamed sources saying police had started to pull multiple bodies from the water.

Witness Ari Schulman described "a stream of sparks" and what looked like a large firework when the collision erupted overhead as he drove home.

"Initially I saw the plane and it looked fine, normal. It was right about to head over land," he told CNN.

"Three seconds later, and at that point it was banked all the way to the right... I could see the underside of it, it was lit up a very bright yellow, and there was a stream of sparks underneath it," Schulman added. "It looked like a Roman candle."

US President Donald Trump said in an official statement that he had been "fully briefed" and said of any victims, "may God bless their souls."

But less than four hours after the disaster – and while other officials stressed they were waiting for investigations to unfold – he went on social media to critique the air traffic control.

"The airplane was on a perfect and routine line of approach to the airport. The helicopter was going straight at the airplane for an extended period of time. It is a CLEAR NIGHT, the lights on the plane were blazing," Trump wrote on his app Truth Social.

"Why didn't the helicopter go up or down, or turn. Why didn't the control tower tell the helicopter what to do instead of asking if they saw the plane. This is a bad situation that looks like it should have been prevented. NOT GOOD!!!"

Dark, near-freezing river

The Federal Aviation Administration ordered the grounding of all planes at Reagan National, and Washington's police said on X that "multiple agencies" were responding to the crash site in the Potomac.

Kristi Noem, the country's new secretary of homeland security, posted on X that she was "deploying every available US Coast Guard resource for search and rescue efforts in this horrific incident at DCA."

Police said fire boats had joined the operation on the river, where any work was complicated by the fact it was dark and close to freezing. Dozens of fire trucks headed toward the airport.

The FAA said a Bombardier regional jet operated by American Airlines subsidiary PSA Airlines "collided in midair" with a Sikorsky UH-60 helicopter as it approached for landing at Reagan at around 9 p.m. (0200 GMT). The plane had left from Wichita, Kansas.

US Senator Roger Marshall of Kansas said on X the collision was "nothing short of a nightmare."

American Airlines' CEO issued a video statement in which he expressed "deep sorrow."

Crowded airspace

Questions were expected to focus on how a passenger plane with modern collision-avoidance technology and nearby traffic controllers could collide with a military aircraft over the nation's capital.

The airspace around Washington is often crowded, with planes coming in low over the city to land at Reagan airport and helicopters – military, civilian and carrying senior politicians – buzzing about both day and night.

The same airport was the scene of a deadly crash in January 1982 when Air Florida flight 90, a Boeing 737, took off but quickly plummeted, hitting the 14th Street bridge and crashing through the ice into the Potomac River. 78 people died.

Investigators concluded the pilot had failed to activate sufficient de-icing procedures.

The last major fatal US crash was in 2009, when Continental Flight 3407 from New Jersey to Buffalo, New York crashed and killed all 49 people aboard.