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The Trump administration plans to loosen constraints on the use of nuclear weapons and develop a new low-yield nuclear warhead for US Trident missiles, according to a former official who has seen the most recent draft of a policy review.

Jon Wolfsthal, who was special assistant to Barack Obama on arms control and nonproliferation, said the new nuclear posture review prepared by the Pentagon, envisages a modified version of the Trident D5 submarine-launched missiles with only part of its normal warhead, with the intention of deterring Russia from using tactical warheads in a conflict in Eastern Europe.

The new nuclear policy is significantly more hawkish that the posture adopted by the Obama administration, which sought to reduce the role of nuclear weapons in US defence. 

The NPR also expands the circumstances in which the US might use its nuclear arsenal, to include a response to a non-nuclear attack that caused mass casualties, or was aimed at critical infrastructure or nuclear command and control sites.

The nuclear posture review (NPR), the first in eight years, is expected to be published after Donald Trump’s State of the Union speech at the end of January. 

Wolfsthal, who has reviewed what he understands to be the final draft of the review, said it states that the US will start work on reintroducing a sea-launched nuclear cruise missile, as a counter to a new ground-launched cruise missile the US has accused Russia of developing in violation of the 1987 Intermediate-Range Nuclear Forces (INF) treaty. Wolfsthal said that earlier drafts of the NPR was even more hawkish. The final draft drops proposals to develop a nuclear hyper-glide weapon, and to remove assurances to non nuclear weapons states that the US will not use its nuclear arsenal against them. 

“My read is this is a walk-back from how extreme it was early on. It doesn’t have as much terrible stuff in it as it did originally,” Wolfsthal said. “But it’s still bad.”

“What I’ve been told by the people who wrote the thing was what they were trying to do was to send a clear deterrent message to Russians, the North Korean and the Chinese. And there is pretty good, moderate but strong language that makes clear that any attempt by Russia or North Korea to use nuclear weapons would result in a massive consequence for them and I think that’s actually moderate, centrist and probably very much needed.”

“Where they go overboard, is where they say that in order to make that credible the US needs to develop two new types of nuclear weapons,” he added.

Wolfsthal said the modified Trident warhead, with just the primary (fission) part of its thermonuclear warhead, was “totally unnecessary” as the US already has low-yield weapons, B61 gravity bombs and air-launched cruise missiles, in its arsenal. 

He also said it was “pretty dumb” to put a low-yield “tactical” weapon on the planned new Columbia-class ballistic missile submarines, because firing it would give away a submarine’s position.

“We spend $5bn per submarine to make it invisible and we put a lot of warheads on each submarine and so what they want to do is take one missile, put one small warhead on it and launch it first, so the submarine is vulnerable to Russian attack.” Wolfsthal said. “That strikes me as being unsustainable from a naval strategy point of view.”

The development of a low-yield warhead for a sea-launched ballistic missile is based on the belief that in any conflict with Russia on Nato’s eastern flank, the Russians would use a tactical nuclear weapon early on, to compensate for their relative weakness in conventional arms. The Russians, the argument goes, would count on US reluctance to use the massive warheads on its existing weapons, leading Washington to back down. 

Hans Kristensen, the director of the nuclear information project at the Federation of American Scientists, said that justification for developing the new weapons was incoherent. 

“It assumes that the intelligence community has determined that one or several adversaries out there are gambling that the US would be self-deterred from using a ballistic missile warhead because they have larger yield. Thats just not the case. We have never, ever heard anyone say that is so,” Kristensen said. 

“I don’t think any adversary – certainly not Russia, – would gamble that if they did something with nukes that were low yield that we would not respond. That’s completely ludicrous,” he added. “I think this is about having some warhead work at the laboratories and exploring options. I don’t see this as a real mission.”

Daryl Kimball, the head of the Arms Control Association, said that the development of new weapons in the US nuclear arsenal was “dangerous, Cold War thinking”. 

“The United States already possesses a diverse array of nuclear capabilities, and there is no evidence that more usable weapons will strengthen deterrence of adversaries or compel them to make different choices about their arsenals,” Kimball wrote on the Arms Control Today website.

He also cautioned against moves to broaden the circumstances in which nuclear weapons would be used.

“The use of even a small number of these weapons would be catastrophic,” Kimball said. “Threatening nuclear attack to counter new kinds of ‘asymmetric’ threats is unnecessary, would increase the risk of nuclear weapons use, and would make it easier for other countries to justify excessive roles for nuclear weapons in their policies.”

[Julian Borger is the Guardian's world affairs editor. He is the author of The Butcher's Trail, a book on the pursuit and capture of the Balkan war criminals, published by Other Press.]