"The threat of the use, intentional or otherwise, of nuclear weapons is growing," the UN's representative for disarmament affairs, Izumi Nakamitsu, told a preliminary review meeting of the Nuclear Non-Proliferation Treaty (NPT).

The United States, which holds one of the world's largest nuclear arsenals, also warned the conference that the prospects for progress on disarmament was currently "bleak".

The NPT, introduced at the height of the Cold War a half century ago, seeks to prevent the spread of atomic weapons while putting the onus on nuclear states to reduce their stockpiles.

Speaking at the opening of the Geneva meeting, Nakamitsu warned that "the world today faces similar challenges to the context that gave birth to the NPT."

The NPT treaty, which counts 191 state parties, faces a comprehensive review every five years, with preparatory committees each year in between.

The next full review of the treaty is scheduled for 2020.

The year's meeting comes after North Korea, which pulled out of the treaty 15 years ago, declared a moratorium on nuclear and long-range missile tests and said it would dismantle its nuclear test site.

Nakamitsu hailed the announcement, voicing hope that the move "will contribute to building trust and to sustaining an atmosphere for sincere dialogue and negotiations."

- 'Keep pressure' on Pyongyang -

Other speakers at the opening of the Geneva meeting, including the European Union representative, stressed the need to "keep pressure" on North Korea.

And Christopher Ford, US Assistant Secretary for International Security and Nonproliferation, insisted that Pyongyang had "yet to return to compliance" of the NPT.

North Korea's nuclear programme was one reason why "the nonproliferation regime today faces great threats," he said, also pointing to Iran's nuclear programme.

Ford said the NPT treaty had made great strides over the past half century to avoid proliferation and ward off the use of nuclear weapons, but warned that "deteriorating security conditions have made near-term prospects for further progress on disarmament bleak."

Nakamitsu also cautioned that the overall "geopolitical environment is deteriorating."

"Some of the most important instruments and agreements that comprise our collective security framework are being eroded," she said.

"Rhetoric about the necessity and utility of nuclear weapons is on the rise," she said, stressing that "modernisation programmes by nuclear-weapons states are leading to what many see as a new, qualitative arms race."

Nakamitsu noted that until recently all the major powers have been engaged in "continuous and successive negotiations on arms control and disarmament."

"Yet not only have we seen an unfortunate hiatus in these efforts, there are real concerns that unless we reverse this trend we will soon be back in a situation for the first time in which there are no verified constraints on nuclear arsenals," she said.

Five of the world's nine nuclear-armed states -- Britain, China, France, Russia, and the United States -- are parties to the NPT.

India and Pakistan, as well as Israel, which has never acknowledged it has nuclear weapons, have never signed the treaty.

But despite their treaty obligations, observers say that all nuclear-armed NPT members are engaged in modernising their arsenals and making nuclear weapons a more central part of their defence strategies.

President Donald Trump's administration has for instance recently decided to upgrade the US nuclear weapons arsenal and to complement massive "strategic" bombs with smaller "tactical" weapons, in a move critics say would make them easier to use.