The Trump administration has announced a new policy to modernise its nuclear arsenal by developing new smaller atomic bombs and enhancing its deterrence capabilities.

The strategy, President Donald Trump said, is tailored and flexible to address the wide array of threats in the 21st century.

"It pursues modernisation of our nuclear command, control, and communications, all three legs of our triad, our dual capable aircraft, and our nuclear infrastructure," said Trump after the the 2018 Nuclear Posture Review (NPR) was out at the Pentagon.

The strategy develops capabilities aimed at making use of nuclear weapons less likely, he said, adding that it enhances deterrence of strategic attacks against the US, its allies and partners, that may not come in the form of nuclear weapons.

"And, importantly, it reaffirms our commitment to arms control and nuclear non-proliferation, maintains the moratorium on nuclear testing, and commits to improving efforts to prevent, detect, and respond to nuclear terrorism," Trump said.

In his preface to the new policy, that runs into 100 pages, Defence Secretary Jim Mattis said maintaining an effective nuclear deterrent is much less expensive than fighting a war that the US was unable to deter.

"While we will be relentless in ensuring our nuclear capabilities are effective, the US is not turning away from its long-held arms control, non-proliferation, and nuclear security objectives," Mattis said.

"Our commitment to the goals of the treaty on the NonProliferation of Nuclear Weapons (NPT) remains strong," he said.

Mattis said Russia and China are pursuing entirely new nuclear capabilities tailored to achieve particular national security objectives while also modernising its conventional military, challenging traditional US military superiority.

Russia, Mattis said, is modernising these weapons as well as its other strategic systems.

"Even more troubling has been Russia's adoption of military strategies and capabilities that rely on nuclear escalation for their success," said Mattis.

"These developments, coupled with Russia's seizure of Crimea and nuclear threats against our allies, mark Moscow's decided return to Great Power competition," he said.

North Korea and Iran are the other two countries that figure in the report.

There is no mention of Pakistan in the report, even as senior US officials in the past have expressed concerns over the security of nuclear weapons in the South Asian country.

"Expanding flexible US nuclear options now, to include low-yield options, is important for the preservation of credible deterrence against regional aggression," said the report as released by the Pentagon.

"It will raise the nuclear threshold and help ensure that potential adversaries perceive no possible advantage in limited nuclear escalation, making nuclear employment less likely," the report said.

The US will maintain, and enhance as necessary, the capability to forward deploy nuclear bombers and dual capability aircraft (DCA) around the world, it said.

Additionally, in the near-term, the US will modify a small number of existing SLBM warheads to provide a low-yield option, and in the longer term, pursue a modern nuclear-armed sea-launched cruise missile (SLCM), the report said.

For the longer term, the US will pursue a nuclear-armed SLCM, leveraging existing technologies to help ensure its cost effectiveness, according to the report.

"SLCM will provide a needed non-strategic regional presence, an assured response capability. It also will provide an arms control compliant response to Russia's non-compliance with the Intermediate-range Nuclear Forces Treaty, its nonstrategic nuclear arsenal, and its other destabilising behaviours," the report said.

The Nuclear report also said the US will not use or threaten to use nuclear weapons against non-nuclear weapons states that are party to the NPT and in compliance with their nuclear nonproliferation obligations.

Following the release of the report, US Ambassador to the UN, Nikki Haley, said the NPR makes sure the US remains flexible and well prepared for the unique threats it faces today.

"We want to see a world that is free of nuclear weapons, but our nuclear policy needs to be rooted in the reality of the world we live in, where aggressive regimes like North Korea threaten us and our allies with their pursuit of illegal nuclear and ballistic weapons," said Haley.

"While we have continued to reduce the size of our nuclear arsenal, countries like Russia and China have moved in the opposite direction. The NPR appropriately addresses the nature of these threats and ensures that the safety and security of the American people remain our top priority," she said.

In a special briefing with reporters at the Pentagon, Under Secretary of State for Political Affairs, Tom Shannon, said that in addition to China and Russia, other "unfriendly regimes and enemies of the US put lives at risk by pursuing nuclear weapons".

"North Korea continues its illicit nuclear weapons programme and missile capabilities. Iran retains the technological capability to develop a nuclear weapon within one year of deciding to do so," Shannon said.

He said that nuclear terrorism is still a major threat in the 21st century and countries need to work to mitigate it.

"The potential threat of non-state actors getting their hands on a nuclear weapon remains at the front of all of our minds," Shannon said.

"Because of the dangerous world we live in and our unwavering commitment to our allies, the 2018 NPR focuses on strengthening extended deterrence," he said.

The US has formal extended deterrence commitments that assure European, Asian and Pacific allies of its commitment to use nuclear force to protect them, if necessary.

Ongoing, close collaboration with allies and partners is essential to deterring or defeating the common threats they face, said Shannon, noting that this collaboration includes sustained dialogues and joint military exercises.

"But we also realise that every ally and partner faces a different threat environment. We will continue to work with them to tailor our assurance strategies in ways that are most effective for their specific situation," he said.

Shannon said the US remains committed to the allies under its nuclear umbrella.

"Our extended deterrence commitments are unwavering. We have the ability and will to fulfil them. Potential adversaries should not doubt our resolve," Shannon said.