The SIPRI think tank estimates that China has a stockpile of around 350 nuclear warheads -- a small sum when compared with the United States and Russia.

But it is growing fast, and the country could have 1,500 warheads by 2035, according to a Pentagon estimate published in November.

Concerns about the build-up have been growing in the West, and the G7 leaders warned the expansion "without transparency nor meaningful dialogue poses a concern to global and regional stability".

Since its first nuclear test in 1964, China has been content to maintain a comparatively modest arsenal and has maintained that it will never be the first to use nuclear weapons in a conflict.

But in recent years, under President Xi Jinping, it has begun a massive military modernisation drive that includes upgrading its nuclear weapons to not only deter foes but also be able to counter-attack if deterrence fails.

In April, foreign ministers from G7 wealthy democracies also warned over the expansion of China's nuclear capacity, urging "strategic risk reduction discussions" with Washington and greater transparency from Beijing.

G7 leaders, who earlier Friday laid wreaths at a memorial to the estimated 140,000 people killed in the nuclear bombing of Hiroshima, also took aim at Russia.

They condemned its "irresponsible nuclear rhetoric" and called a plan to station nuclear weapons in Belarus "dangerous and unacceptable".

They also slammed proliferation, warning North Korea against "provocative actions" and urging Iran to "cease nuclear escalations".

The document is the first time a G7 summit has produced a leaders' statement focused on nuclear disarmament, a reflection of efforts by Japan's Prime Minister Fumio Kishida, who is from Hiroshima.

He has sought to push the issue up the agenda at the three-day talks, and earlier led the leaders around the Hiroshima peace museum, where they confronted evidence of the suffering inflicted by the August 6, 1945 US nuclear attack.

The leaders reiterated their commitment to achieving a world without nuclear weapons "with undiminished security for all", a caveat that nods to the difficulty of achieving nuclear disarmament progress in the current global security climate.

"Achieving the world we hope to see requires a global effort to take us from the harsh reality to the ideal, no matter how narrow the path may be," the leaders said, without offering concrete commitments of their own.

Three G7 members -- the United States, Britain and France -- have nuclear weapons, and the rest are protected by the US "nuclear umbrella".