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

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

But 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 preparatory committee in advance of the 2020 NPT review conference 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."

She warned however 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.

But despite their commitments under the treaty, they are all engaged in modernising their arsenals and making nuclear weapons a more central part of their defence strategies.

The administration of President Donald Trump 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.