The heavy bomber was on a routine testing mission with a mixture of military, government and civilian contractors on board when it came down in a huge fireball at Edwards Air Force Base, 60 miles (95 kilometers) north of Los Angeles.

Footage of the aftermath of the crash, which officials said was "unsurvivable," showed a large charred patch of ground on which almost nothing remained of the huge plane.

"Edwards Air Force Base experienced a horrible tragedy, and we lost eight great Americans," Colonel James Hayes told reporters at the base.

Hayes said the B-52 Stratofortress -- a long-range bomber used by the US military since the 1950s -- was on a test sortie as part of a radar modernization process.

"It took off, and immediately after takeoff, crashed and burst into flames," he said, adding emergency services quickly swung into action, but soon determined that there would be no one to rescue.

"After reviewing the footage of the crash, it was deemed that this was an unrecoverable crash and unsurvivable."

There was no immediate indication as to the cause of the tragedy, and a safety investigation probe was immediately begun.

The crash happened around lunchtime at the base, a major operations center for the US military, and was contained within the perimeter, Hayes told reporters.

In the aftermath of the accident, the airfield was closed and all inbound aircraft were being diverted, the base said on social media.

Hayes said the identities of those who perished would not be released until all next of kin had been informed, a process he said was ongoing and could last for the rest of the day.

The B-52 is a heavy bomber that first flew in 1954 and was originally designed for war with the Soviet Union. It has received continual upgrades to keep it in service for decades since the Cold War's end.

The massive bomber -- which can carry a range of weapons, including bombs and cruise missiles -- has a wingspan of 185 feet (56 meters) and a length of 159 feet (48 meters).

The plane is usually crewed by five people: an aircraft commander, a pilot, a radar navigator, a navigator and an electronic warfare officer, according to a US Air Force fact sheet on the plane.

With a combat range of up to 8,800 miles, the plane is capable of carrying a nuclear payload.

The United States has deployed the aircraft in conflicts in Vietnam, the Gulf, Iraq, Afghanistan, and most recently in Iran.