Editor's Note: This article originally appeared on Business Insider.

The B-52 bomber, a critical component of the U.S. strategic bomber fleet, is no longer approved to carry nuclear gravity bombs.

The latest update to the US Air Force “Safety Rules for U.S. Strategic Bomber Aircraft” instructional guidance identifies the “removal of B61-7 and B83-1 from B-52H approved weapons configuration” as a change to the previous version.

The change was first noticed by nuclear expert Hans Kristensen and first reported The Drive, and it comes amid concerns the bomber isn't able to penetrate enemy defenses and deliver such a strike.

The B61 is the main thermonuclear gravity bomb in the U.S. arsenal, and the B83 is the US nuclear gravity bomb with the largest explosive yield. Only the stealthy flying wing B-2A Spirit will carry these weapons.

“The B-52 remains an instrument of national security and a universally recognized symbol of American airpower, able to deliver the widest variety of stand-off and direct-attack nuclear and conventional weapons for the Nation,” Global Strike Command told Insider in a statement Thursday.

The command further explained that “as the nature of modern warfare has changed, so have our tactics and weapons,” adding that “as a natural progression in the development of the airframe, B-52 crews do not currently train to employ the B83 and B61.”

An inert training version of a B61 in an underground Weapons Storage and Security System vault at Volkel Air Base in the Netherlands.(U.S. Air Force photo)

The B-52H, the latest variant of the 1950s bomber, carries AGB-86B air-launched cruise missiles, which are expected to eventually be replaced by the Long-Range Stand Off (LRSO) weapon.

Gen. John Hyten, current Vice Chairman of the Joint Chiefs of Staff and former head of U.S. Strategic Command, said in 2017 that without LRSO, “you don't have the B-52 as a viable platform” — a statement reflecting the need for stand-off capabilities.

While the B-52 has been receiving a number of upgrades to keep it flying until at least 2050, the long-range bomber remains vulnerable to modern strategic air defenses. Unable to conduct penetrating strikes, stand-off is necessary to keep the bomber in the fight.

Kristensen, director of the Nuclear Information Project at the Federation of American Scientists, first noted nearly two years ago that the B-52H was likely no longer carrying nuclear gravity bombs.

He found that the National Nuclear Security Administration had stopped listing the B-52H as a combat platform capable of delivering nuclear gravity bombs. He further reported that U.S. Strategic Command had not been assigning nuclear gravity bombs to B-52s since at least 2010.

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