At least six people including crew members were killed when two World-War-II-era planes collided in mid-air at a show in Dallas, a pilots association said, with witness footage showing Saturday's crash ending in a fiery explosion on the ground.

By early Sunday it was still unclear exactly how many people were in the two aircraft, a Boeing B-17 Flying Fortress and a smaller Bell P-63 Kingcobra, the Federal Aviation Administration said.

But the Allied Pilots Association, the collective bargaining agent for American Airlines, confirmed two of its retired members died in the Texas accident.

"Our hearts go out to their families, friends, and colleagues past and present," the group said late Saturday in a statement on Twitter.

It was not yet known whether anyone survived the afternoon crash, which occurred during the Wings Over Dallas Airshow at Dallas Executive Airport.

Dallas Mayor Eric Johnson called the incident "a terrible tragedy," and said on Twitter that "no spectators or others on the ground were reported injured."

Hank Coates, the chief executive of the Commemorative Air Force (CAF) whose planes were involved in the accident, said the B-17 "normally has a crew of four or five," while the P-63 is operated by a single-pilot.

He said the Sunday part of the air show was cancelled.

Multiple videos posted on social media showed dramatic scenes of the smaller plane descending towards the lower-flying B-17 and crashing into it.

After the collision, the planes appeared to break apart into several large pieces before crashing to the ground and exploding in a ball of fire, creating a huge plume of black smoke.

The crash scattered debris across the airport grounds as well as on a nearby highway and strip mall, Johnson said.

The FAA said its agents and the National Transportation Safety Board would investigate the incident.

The CAF's Coates said the pilots who operate the planes during such shows are experienced volunteers with "very thorough training" and are often retired military pilots.

The CAF has some 180 vintage aircraft that fly roughly 6,500 hours per year in air shows, tributes to veterans, and training flights, according to Coates.

The B-17, a four-engined bomber, played a major role in winning the air war against Germany in World War II. With a workhorse reputation, it became one of the most produced bombers ever.

The P-63 Kingcobra was a fighter aircraft developed during the same war by Bell Aircraft but it was used in combat only by the Soviet Air Force.

One of the last major crashes of a B-17 was on October 2, 2019, when seven people died in an accident at an airport in Windsor Locks, Connecticut.