The Trump administration’s Nuclear Posture Review is due to be released at the end of this month, but even before its official unveiling, a report on a draft of the policy document has some critics up in arms.

Defense Secretary Jim Mattis has already confirmed publicly that the Trump nuclear policy will maintain all three legs of the nuclear triad: bombers, submarines, and land-based intercontinental ballistic missiles.

But what has some experts worried are reported plans to begin work on smaller yield versions of existing weapons and at least one new delivery system, a sea-based nuclear-tipped cruise missile. It would be a particularly destabilizing weapon because it cannot be distinguished from a conventional cruise missile upon launch.

“The NPR is taking the nation in exactly the wrong direction. It endorses a massive trillion-dollar nuclear build up and a suite of more nuclear missions and more ‘usable’ nuclear weapons to fulfill these missions,” said Joe Cirincione, who heads the Ploughshares Fund, an arms control advocacy group. “It brings us closer to nuclear war.”

Jon Wolfsthal, a former Obama administration official, told the Guardian newspaper that he has seen the most recent draft of the review, and that it includes a plan to develop a low-yield warhead for the Trident D5, launched by ballistic missile submarines.

The Pentagon insists it's not building a new generation of nuclear weapons, but is updating and improving the old ones.

One example is the B-61 gravity bomb. The old version, designed during the Cold War, was unguided and had vacuum tubes.The new version, which was authorized by President Barack Obama, has tailfins that can more precisely guide the bomb to a target. It also has a “dial-a-yield” feature that can reduce the blast radius to a small enough area that it can be considered a tactical or “battlefield” weapon.

Critics say that makes the bomb more acceptable and therefore makes it more likely it could be used in some scenarios, such as attacking deeply-buried hardened targets, or to deliver a shock attack that would be designed to quickly end a conflict.

But Pentagon officials who have asked not to be identified because they are not authorized to speak publicly on nuclear weapons policy argue that by making nuclear weapons more usable they are more credible as a threat, and therefore increase the deterrence value of the U.S. nuclear arsenal.

The logic goes something like this: If North Korea’s Kim Jong Un thinks the only nuclear option is a powerful bomb that could not only destroy his country but spread a cloud of radioactivity over South Korea and Japan, then the threat of a nuclear attack seems less credible.

But if the U.S. has low-yield tactical weapons that could take out a nuclear facility without causing more widespread destruction or massive deaths, its use seems more credible.

It’s the paradox of nuclear deterrence. If you believe, as the U.S. does, that the best use of nuclear weapons is for them to never to be used, then they have to be seen as able to be used, or their deterrence value is diminished.

So the fact that they can be used makes them less likely they will be used, the officials argue.