With nuclear-armed nations expanding and modernizing their arsenals, the UN chief called for a revitalized push to reduce and eventually eliminate those weapons.

"A worrisome new arms race is brewing. The number of nuclear weapons could rise for the first time in decades," Guterres told the General Assembly on the final day of its yearly session.

"Any use of a nuclear weapon - anytime, anywhere, and in any context - would unleash a humanitarian catastrophe of epic proportions," he said.

"Nuclear sabers are again being rattled. This is madness. We must reverse course," he said.

Russia and the US have by far the largest arsenals, but China's has been growing quickly. North Korea has also defied the world with its nuclear program and repeated missile tests.

In its own speech, one of the last of the week-long marathon of September's UN General Assembly, North Korea accused arch-rival the US of driving the peninsula "closer to the brink of nuclear war."