WASHINGTON - The US plan to deploy new low-yield nuclear weapons is aimed at convincing Russia to respect existing agreements on limiting the weapons, Secretary of Defense James Mattis said Tuesday.

Mattis told a hearing in Congress that Moscow is violating the Intermediate Nuclear Forces Treaty and Washington want to pressure it to get back in line.

"As you know, we have an ongoing issue with Russia 's violation of the INF. I want to make certain that our negotiators have something to negotiate with, that we want Russia back into compliance," he told the House Armed Services Committee.

"We are going to stay inside the INF-compliant requirements but we are going to do research and development of an alternative weapon that should put Russia in a position to see the value to returning to being INF-compliant," he said.

For several months Washington has accused Russia of developing a new intermediate-range, ground-launched cruise missile that violates the treaty signed by Ronald Reagan and Mikhail Gorbachev and implemented in 1988.

US defends controversial

new nuclear policy at UN

The United States defended its controversial new nuclear policy at the United Nations on Tuesday, insisting a more assertive stance was needed to confront a worsening security climate.

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."