The Pentagon's so-called Nuclear Posture Review (NPR), unveiled last week, outlined US plans to revamp its nuclear arsenal by developing new low-yield atomic weapons.

Washington has portrayed the policy as a necessary response to actions taken by Russia and China, claims Moscow and Beijing have fiercely denounced.

"Today's security environment is more dynamic, complex, demanding, and threatening than any since the end of the Cold War", the US ambassador to the UN's conference on disarmament, Robert Wood, said as he unveiled the NPR at the Geneva-based body.

He accused Russia, China and North Korea of boosting their nuclear stockpiles and "raising the prominence of nuclear weapons in their security strategies".

"Some in that room may think that we should just put our heads in the sand and ignore the threats that are being faced out there and just let Russia and China and North Korea continue to do what they are doing," Wood told reporters outside the conference hall.

"This NPR reflects the reality of the security situation", he added. "It is important to strengthen nuclear deterrence."

Moscow has called the new US policy "bellicose" and "anti-Russian" and warned it might take responsive measures to boost its own security.

China has said that Washington's assessments of its nuclear intentions amounted to "wild guesses", while Iran charged the US with bringing the world "closer to annihilation".

The new NPR is the first time since 2010 that the US military has spelled out how it foresees nuclear threats in the coming decades.

It has triggered accusations that President Donald Trump's administration was seeking to lower the threshold needed for a nuclear strike and breaching non-proliferation agreements.

"The nuclear threshold is not being lowered", Wood said.

"Our intention is to reduce the risk that others might miscalculate or gamble that they have some exploitable advantage. The objective is to make clear it is not in others' interest to use nuclear weapons."