US President Donald Trump has commented on last night's tragedy in which 28 people died in the collision of an American Airlines passenger plane and a US military Black Hawk helicopter.

The plane and helicopter collided in mid-air and crashed into the Potomac River near Washington's Reagan National Airport.

"The helicopter was in the wrong place at the wrong time and a tragedy occurred," Trump said at a White House press conference on Thursday.

He said the country was "in mourning," as he hailed the search and rescue operation at all levels and declared that there were no survivors.

"This has really shocked a lot of people, including people from other nations, unfortunately," Trump said.

He added that there were Russians on the plane, but did not mention other nationalities. He said he has "ideas" and theories about what happened, although he acknowledged that the investigation is still in the early stages.

"We didn't know what led to the accident, but we have strong thoughts and ideas... We will find out how this disaster happened and make sure nothing like this ever happens again," the president added.

Trump also attacked his political opponents, this time Barack Obama, for allegedly hiring "mediocre" personnel for air traffic control jobs that require "high-level intelligence," the BBC reports.

The passenger plane involved in a deadly crash near Washington took off from Wichita, Kansas, without any problems, the city's airports director, Jesse Romo, said.

"To our knowledge, everything went normally," Romo told reporters during a press conference, adding: "So there were no anomalies in the operation."

People planning to fly from Wichita to Reagan National Airport this week should check with American Airlines for the current situation, Romo added.

Congressman Ron Estes of Wichita commented on the route taken by the American Airlines flight from Wichita, Kansas, to Washington. He confirmed that the route has been operating for a year. He said that landing at the airport requires skilled pilots, as there are "turns" as they leave the airport.

Wichita Mayor Lily Wu said at a press conference that there are no survivors and that 28 bodies have been recovered.

"This is a terrible tragedy that will forever unite those in Washington and Wichita," she said.

American Airlines CEO Robert Isom said the two pilots on the plane were experienced and had many hours of flying time.