By Idrees Ali

WASHINGTON (Reuters) – Concerned about Russia’s growing tactical nuclear weapons, the United States will expand its nuclear capabilities, a policy document released on Friday said, a move some critics say could increase the risk of miscalculation between the two countries.

It represents the latest sign of hardening resolve by President Donald Trump’s administration to address challenges from Russia, at the same time he is pushing for improved ties with Moscow to rein in a nuclear North Korea.

The focus on Russia is in line with the Pentagon shifting priorities from the fight against Islamist militants to “great power competition” with Moscow and Beijing.

“Our strategy will ensure Russia understands that any use of nuclear weapons, however limited, is unacceptable,” the document, known as the Nuclear Posture Review, said.

The rationale for building up new nuclear capabilities, U.S. officials said, is that Russia currently perceives the United States’ nuclear posture and capabilities as inadequate.

By expanding its own low-yield nuclear capability, the United States would deter Russia from using nuclear weapons, U.S. officials argue.

Low-yield nuclear weapons, while still devastating, have a strength of less than 20 kilotons. The atomic bomb dropped on Hiroshima had about the same explosive power.

The argument for these weapons is that larger nuclear bombs are so catastrophic that they would never be used and do not work as an effective deterrent. With less power and destruction, the low-yield option would potentially be more likely to be used, serving as an effective deterrent.

The Pentagon document, which is largely in line with the previous review in 2010, said the U.S. will modify a small number of submarine-launched ballistic missile warheads with low-yield options.

In the long term, the U.S. military will also develop a new nuclear-armed sea-launched cruise missile. The missile could have the less powerful option, but a decision has not been made, and will take up to a decade to develop, officials said.

Greg Weaver, deputy director of strategic capabilities at the Pentagon, said the United States would be willing to limit developing the missile if Russia would “redress the imbalance in non-strategic nuclear forces.”

Weaver said the most difficult task for those working on the review was trying to address the gap between Russian and American non-strategic nuclear weapons.

Russia has a stockpile of 2,000 non-strategic nuclear weapons, according to the Pentagon. The U.S. has a few hundred active low-yield weapons deployed in Europe.

RESPONDING TO RUSSIA

U.S. officials argue that since the last nuclear review, Russia has expanded and modernized its non-strategic nuclear weapons, annexed Crimea in 2014, and deployed a ground-launched cruise missile that breaches the 1987 Intermediate-range Nuclear Forces (INF) treaty. The treaty bans testing and fielding missiles with ranges of 500-5,500 kms (310-3,417 miles).

“The U.S. is not arms racing, we are responding to Russian initiative here,” Weaver said.

Some experts have questioned the expansion.

Jon Wolfsthal, a former top advisor to President Barack Obama on arms control, said there was a possibility that it could lead to a miscalculation.

“If we put nuclear weapons on cruise missiles and we launch conventional cruise missiles, how does Russia know that they are conventional?” he said.

The document argues that by developing U.S. nuclear responses, it raises the Russian threshold for using the weapons, rather than lowering the U.S. threshold.

Kingston Reif, director for disarmament research at the Arms Control Association advocacy group, said the document could bolster a new kind of arms race.

“It’s not an arms race in terms of numbers like during the Cold War, but is an arms race that involves more than just the United States and Russia and it involves upgrading and improving the capability of existing nuclear forces,” Reif said.

The review called for continuing the B-83 bomb, the largest nuclear weapon in the U.S. stockpile, until a replacement is found, reversing plans to retire it.

(Reporting by Idrees Ali; Editing by Yara Bayoumy and James Dalgleish)