A towering pall of black smoke billowing from the crash site was visible for miles immediately ​after the accident.

He said the "mixed crew" aboard the aircraft consisted of government civilians, government contractors and uniformed military personnel. ⁠Aerospace giant Boeing which designed and built the plane, said two of its employees were among the dead.

The flight was intended to support ​a radar modernization program, Hayes told reporters. The cause of the crash was unknown and under investigation, he added.

Air Force officials did not name the ​victims, saying they were still in the process of notifying their next of kin.

Aerial video footage of the crash scene, about 161 km north of Los Angeles, showed a charred, smoldering patch of the desert floor larger than a football field as an emergency vehicle was seen driving along the site's perimeter. From a distance, ​there were no large pieces of debris readily visible in the footage.

Hayes said the crash was quickly "deemed to be unsurvivable."

Because of damage to the ​runway, he said, "we're grounding all operations at Edwards Air Force Base" through at least Tuesday, adding that no operations beyond the base would be suspended.

Edwards, a sprawling ‌test ⁠flight facility established in the 1930s around a dry lake bed, occupies about 1,245 square km of the Mojave desert, making it the Air Force's largest airfield.

Its experimental aviation legacy includes the flight by Chuck Yeager in the Bell X-1 aircraft that broke the sound barrier in 1947, test flights of the X-15 aircraft and the first landings of NASA's space shuttles.

BACKBONE OF BOMBER FORCE 

The B-52 Stratofortress, a long-range, subsonic aircraft built ​to carry up to 31,750kg of weapons and supplies, has ⁠long served as the backbone of the US crewed strategic bomber force, according to the military.

The swept-wing aircraft is capable of unleashing the widest range of weapons in the US inventory, from cluster bombs and gravity ​bombs to precision-guided missiles and nuclear warheads, at altitudes of up to 15,166m, according to an ​Air Force fact ⁠sheet. Its combat range extends more than 12,800km without refueling.

Monday's incident marked the first crash of a B-52 Stratofortress since the same type of bomber crashed on the island of Guam in May 2016, according to the Bureau of Aircraft Accidents Archives, a Geneva-based organization that collects global aviation accident data. ⁠All seven ​crew members aboard that aircraft survived.

Only H models of the B-52 remain in the ​Air Force inventory.

The aircraft involved in Monday's crash was assigned to the 412th Test Wing, which is based at Edwards. Most B-52s are stationed in North Dakota and Louisiana.