"We are concerned with China's and Russia's significant nuclear weapons build-up and modernization," the G7 Non-proliferation Directors Group said in a statement published on the French foreign ministry website.

The group brings together the diplomats from the G7 countries -- Britain, Canada, France, Germany, Italy, Japan and the US -- who lead work on limiting the spread of nuclear arms.

Their statement comes ahead of the Monday opening of a month-long New York conference to revise the NPT, under the shadow of intensifying international competition and multiple crises with nuclear dimensions such as the wars in Iran and Ukraine.

"We commit to working with all States Parties to achieve a successful Review Conference in 2026 and to pursue the broadest possible consensus on measures to reinforce the NPT regime across all its three pillars," the G7 negotiators said -- referring to disarmament, non-proliferation and peaceful use of atomic energy.

The group added that they "strongly encourage the United States' pursuit of multilateral strategic stability."

US President Donald Trump has frequently expressed hostility and suspicion towards major multilateral treaties.

But he has backed the idea of a three-way deal between the US, Russia and China -- as well as threatening to resume nuclear testing and accusing Beijing of carrying out secret weapons trials.

The NPT conference will seek consensus to keep the broader treaty alive.

Two previous meetings saw the 191 signatories fail to agree a final text, with observers expecting a repeat at this year's event.