The Pentagon is putting the finishing touches on the first comprehensive review of US nuclear forces in nearly eight years. It's shaping up as President Donald Trump's signature nuclear weapons initiative in the face of a growing North Korean nuclear threat.

The review, which may allow Trump to put his mark on the nuclear inventory for decades to come, could lead to more than $1 trillion in spending over nearly 30 years.

There have been three such reviews since the end of the Cold War, the most recent in 2010 under President Barack Obama.

'This Nuclear Posture Review is unique in that it is the first one to occur after a notable negative evolution in the security environment,' one defense official said. 'Threats have become more stark and volatile since 2010, and a fresh look at nuclear posture is essential to countering these threats.'

Trump ordered the review in a presidential memorandum last January 27, a week after he took office. The memo called for a Pentagon review 'to ensure that the United States nuclear deterrent is modern, robust, flexible, resilient, ready, and appropriately tailored to deter 21st-century threats and reassure our allies.'

At the same time, a review of ballistic missile defenses is underway to identify possible improvements. One initiative that could be part of that is the development of a missile defense system that could detect, track and destroy multiple incoming enemy warheads with the launch of a single US missile.

The reviews are likely to be unveiled just after the President's State of the Union address January 30 and might be mentioned by him in that speech as part of his national security priorities for this year, according to several US defense officials.

The nuclear review is expected to focus on deterrence and reflect the greater threat from North Korea, which has stepped up its testing of missiles and nuclear devices over the last year, as well as an increased focus by Russia on its nuclear inventory.

Congress has recently called for the Pentagon to begin developing a road-mobile ground-launched cruise missile that could violate arms control treaties if it were deployed. For now, it's seen as a potential deterrent to what the US views as Russia's violation of the Intermediate-Range Nuclear Forces Treaty by deploying new missiles capable of firing nuclear warheads.

The review is looking at current needs and capabilities across the US nuclear enterprise, including nuclear laboratories, stockpiles and manufacturing facilities. It also is studying future needs for modernizing aging nuclear weapons, including missiles, submarines and bomber aircraft.

The Pentagon's position is that without increased spending the government will be unable to produce and maintain a stockpile for land, sea, and air-launched nuclear weapons. Operations, interim upgrades and full modernization could cost $1.2 trillion, according to an October 2017 report from the Congressional Budget Office. The report estimates the fiscal needs could include:

$313 billion for a new Columbia-class ballistic missile submarine that can fire nuclear missiles from beneath the ocean's surface.

$149 billion for a new silo-based intercontinental ballistic missile and upgraded launch facilities.

$266 billion for the new B-21 stealth bomber aircraft.

Additional funding for weapons laboratories and command-and-control facilities.

The review could also recommend efficiencies and changes aimed at saving money, officials say.

Defense officials say Trump is not expected to call for increases or modernization of the nuclear arsenal that would take the US beyond current arms control agreements. However, experts say things to watch for in the review include possible recommendations to deploy small nuclear bombs abroad closer to anticipated conflict, or to develop more lower-yield nuclear weapons. Experts, however, have worried that these kinds of developments could make the decision by a US president to use nuclear weapons easier.

It is not clear whether the review will recommend returning bombers to a higher alert status, similar to the Cold War era, or call for some type of nuclear weapons testing.

Overall, however, the aim is to signal potential adversaries that using nuclear weapons against the United States or its allies 'will not achieve hoped-for benefits, but will lead to significant costs,' according to the defense official.