Russia is aggressively building up its nuclear forces and is expected to deploy a total force of 8,000 warheads by 2026 along with modernizing deep underground bunkers, according to Pentagon officials.

The 8,000 warheads will include both large strategic warheads and thousands of new low-yield and very low-yield warheads to circumvent arms treaty limits and support Moscow's new doctrine of using nuclear arms early in any conflict.

In addition to expanding its warheads, Russia also is fortifying underground facilities for command and control during a nuclear conflict.

One official said the alarming expansion indicates Russia is preparing to break out of current nuclear forces constraints under arms treaties, including the 2010 New START and 1987 Intermediate-range Nuclear Forces (INF) treaties. Russia violated the INF accord by testing an illegal ground-launched cruise missile.

The new assessment also suggests Russia is planning to blend its conventional forces with nuclear forces in future conflicts, further complicating the use of American nuclear arms as a deterrent to warfare.

The new disclosure on Russia's arms buildup is among some of the details being studied by the Pentagon as part of a major review of U.S. nuclear forces called the Nuclear Posture Review.

The conclusions of the review are expected to be disclosed around the time President Trump delivers his state of union address to a joint session of Congress next month.

Trump last summer called for sharply increasing the size of the U.S. nuclear arsenal during a meeting of national security officials July 20.

"I want modernization and rehabilitation… It's got to be in tip top shape," Trump said of the ageing U.S. arsenal and nuclear forces.

The current posture of U.S. nuclear forces was set by then-President Barack Obama in a 2010 review that called for reducing the role of nuclear weapons and the size of the arsenal.

The curtailment of nuclear forces by Obama was based on assessments—now considered false by many officials—that nuclear threats posed by Russia and other states had been lowered significantly, and that Moscow and Washington were no longer considered enemies.

Thus the Obama administration based its strategic nuclear deterrence and warfare policies on the outdated assumption that the prospect of a U.S.-Russia military confrontation had been reduced sharply.

Since 2010, however, Russia, China, and North Korea have been engaged in steadily building up their forces with new nuclear arms and delivery systems, while Iran remains an outlier that many experts believe will eventually decide to build a nuclear arsenal in the next decade or sooner when restrictions outlined in the international nuclear deal expire.

The Pentagon's new posture review is based in part on a reversal of the outdated Obama-era assessment.

Russia's nuclear forces—new warheads, missiles, bombers, and submarines—are increasing sharply.

The nuclear modernization is regarded as more ominous because it is coupled to Moscow's new strategic doctrine that calls for quickly resorting to nuclear weapons during any conventional conflict.

The 8,000 nuclear warheads will include both large warheads currently deployed on long-range mobile missiles and missile-firing submarines.

What is new are Russia's large and increasing force of smaller nuclear warheads that will be deployed on its new short- and medium-range missiles, including the ground based SSC-8 cruise missile and the SS-N-27 Kalibr anti-ship and land-attack missile.

Additionally, Russia is developing new and innovative nuclear arms that include very low yield nuclear weapons—less than 1 kiloton or the equivalent of 1,000 tons of TNT-that will be delivered atop precision guided missiles.

Russian weapons researchers have been studying these advanced nuclear arms since the late 1990s. They include precision strike nuclear weapons; clean weapons that produce little radiation fallout; pure fusion weapons that do not require a nuclear blast to trigger them; and tailored effects weapons. Special effects arms include neutron bombs that kill with radiation instead of a large blast, electronics-destroying electromagnetic pulse blasts, and X-ray and Gamma ray weapons.

The weapons will range in blast size from yields of 10 tons of TNT, to 1 kiloton.

The small nuclear arms also will be outfitted with satellite guidance so their accuracy is within a few feet and also will be capable of penetrating into the earth before detonating.

China and India also are said to be developing similar low-yield nuclear arms.

The New START treaty, signed by Russia and the United States limits both countries to 1,550 deployed weapons. However, the treaty has no limits on tactical nuclear weapons and thus has spurred Russian development of the smaller weapons.

Russia's tactical nuclear arms stockpile has been estimated to include between 3,300 and 5,700 weapons with estimates as high as 10,000.

The State Department reported in October that the United States has 1,393 deployed strategic warheads, and Russia has deployed 1,561 warheads. The Congressional Research Service reported that the U.S. non-strategic nuclear stockpile includes around 760 weapons, including around 200 deployed in Europe.

James R. Howe, a nuclear weapons expert, said he also estimates Russia will have at least 8,000 warheads in the next six years depending on how many are loaded on missiles.

For example, Russian President Vladimir Putin said in 2012 that Russia would build 400 new intercontinental range missiles by 2022 that would all be outfitted with between six and 10 warheads.

Howe believes Russia in the years ahead will likely deploy a mix of high-yield, medium-yield, and low-yield warheads integrated with cyber, space, defense, and non-nuclear forces capable of evading U.S. defenses and covering all strategic targeting options.

The warheads will be delivered on current missiles and bombers along with four new ICBMs, two new submarine-launched ballistic missiles, and two new bombers, he said.

The buildup of Russian active and passive defenses and space weapons "indicates the Russians have a very different view of the role of nuclear forces; [and] have applied nuclear warhead technology developments so they have weapons which have political and military utility which can be applied consistently with conflict objectives," Howe said.

The developments also signal that Russia is preparing to conduct nuclear war to achieve strategic war aims.

Rather than numbers of warheads, the threat posed by accurate, low-yield warheads indicates Moscow is contemplating a far different nuclear conflict than those Washington is considering and raises questions about U.S. deterrent capabilities, Howe said.

The Pentagon's Defense Science Board, in a report for the new Trump administration made public in January, said the United States should consider building its own force of low-yield nuclear arms. The development would require lifting a congressional ban on new nuclear arms.

The board questioned the Obama administration's assumption that downgrading the role of U.S. nuclear weapons would lead other nations to follow.

"[Defense Department] leadership is renewing its commitment to the nation’s nuclear deterrent, given the relatively recent recognition of the pervasive threat of adversaries’ nuclear capabilities and doctrines," the report said.

"In short, ‘nuclear' still matters, nuclear is in a class of its own, and nuclear cannot be wished away," the report added.

The Russians plan to use low yield nuclear weapons against NATO's better armed conventional forces as part of a doctrine known as "escalate to de-escalate," the report said.

Joint Chiefs of Staff Vice Chairman Gen. Paul Selva, said in a speech last August that the military needs small nuclear arms that do not cause massive casualties.

"If all you have is high-yield weapons to answer a low-yield attack, it’s still a nuclear attack," Selva said. "Answering that with a conventional weapon is likely not going to have the kind of deterrent value as saying, ‘Even if you use a low-yield weapon, we have options to respond,'" he added.

If a president's only options are using high-yield weapons that cause unwanted indiscriminate killing "then we haven’t presented him with an option to respond to a nuclear attack in kind," the Air Force four-star said.

Mark Schneider, a former Pentagon nuclear weapons policymaker, said Russia has had major programs for precision low yield nuclear weapons for years, with Russian press reports asserting that between 50 and 200 low-yield weapons already are deployed on strategic systems, in addition to traditional high-yield warheads.

"My estimate is that Russia will increase its strategic nuclear forces to over three thousand warheads by 2030. By my count they have announced 22 strategic nuclear modernization programs and they say that the new missiles are heavily MIRVed," Schneider said referring to multiple, independently targetable reentry vehicles.

Tactical nuclear warhead levels have been reduced from Cold War levels but still include around 5,000 to 6,000 arms, he said.

For example, Russia's new long-range precision strike cruise missiles all can be outfitted with nuclear warhead and have significant potential to substitute for strategic nuclear forces.

"If you add my two estimates, the total inventory is nearing 8,000 to 9,000 nuclear weapons and it could further increase," Schneider said.

Russia's military has boasted that its current nuclear forces significantly exceed U.S. forces and that could tempt the Russians to launch an aggressive operation against NATO and use their nuclear forces to dissuade the alliance from responding.

A declassified CIA Intelligence Memorandum from August 2000 said Moscow was planning for very-low yield weapons, including arms with tailored radiation outputs.

On the underground complexes, Pentagon officials said two major Russian command-and-control centers, along with several smaller facilities, also are being modernized, including the secret complex known as Kosvinsky Mountain, located in the Ural Mountains about 850 miles east of Moscow.

Kosvinsky is Russia's main, nuclear-survivable command post that includes underground rail system that will be used to transport Russian leaders from Moscow.

Another command post, also about 850 miles from Moscow, is located at Yamantau Mountain in the Urals near Belorets.

Other underground leadership bunkers have been identified by U.S. intelligence at Voronovo, about 46 miles south of Moscow, and at Sharapovo, some 34 miles from Moscow. Both are equipped with underground rail links.

The upgrading of nuclear command and control was announced by Russia's Defense Ministry last year as expected to be deployed by the end of 2016.

The system is known by its Russian acronym IASBU and uses digital signals to send combat orders and to control strategic forces.

Another major worry for Pentagon nuclear policymakers is Russia's development of a high-speed drone submarine to be armed with a large warhead capable of destroying American ports. The drone sub, known as Kanyon by the Pentagon, was first disclosed in a report by the Washington Free Beacon two years ago.

Howe, the nuclear expert, estimates Russia could deploy several 100-megaton armed Kanyons for nuclear "escalation dominance" during a future war.

A Pentagon spokeswoman had no immediate comment.