"We believe strongly that the further spread of such weapons must be prevented," said permanent UN Security Council members China, France, Russia, the UK and United States, adding: "A nuclear war cannot be won and must never be fought."

The statement was issued after the latest review of the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) -- which first came into force in 1970 -- was postponed from its scheduled date of January 4 to later in the year due to the Covid-19 pandemic.

Putting aside current differences that have caused major tensions between both China and Russia and their Western partners, the five world powers said they saw "the avoidance of war between nuclear-weapon states and the reduction of strategic risks as our foremost responsibilities."

"As nuclear use would have far-reaching consequences, we also affirm that nuclear weapons -- for as long as they continue to exist -- should serve defensive purposes, deter aggression, and prevent war," they said according to the English text released by the White House.

The powers added: "We each intend to maintain and further strengthen our national measures to prevent unauthorised or unintended use of nuclear weapons."

The statement comes as tensions between Russia and the United States have reached heights rarely seen since the Cold War over a troop build-up by Moscow close to the Ukrainian border.

That has raised fears that the Kremlin is planning a new attack on its pro-Western neighbour.

The rise of China meanwhile under President Xi Jinping has also raised concerns that tensions with Washington could lead to conflict, notably over the island of Taiwan.

Beijing considers Taiwan part of its territory and has vowed to one day seize it, by force if necessary.

Russia welcomed the declaration by the atomic powers and expressed hope it would reduce global tensions.

"We hope that, in the current difficult conditions of international security, the approval of such a political statement will help reduce the level of international tensions," the Russian foreign ministry said in a statement.

Kremlin spokesman Dmitry Peskov told the RIA Novosti news agency that Moscow still considered a summit between the world's nuclear powers to be "necessary".

The statement also comes as the world powers seek to reach agreement with Iran on reviving the 2015 deal over its controversial nuclear drive, which was rendered moribund by the US walking out of the accord in 2018.

Washington, which has never ruled out military action against Iran, has repeatedly warned time is running out to agree a deal.