UN Secretary-General Antonio Guterres has expressed a renewed sense of crisis about the growing threat nuclear weapons pose.

Guterres' message was delivered to the Hiroshima Peace Memorial Ceremony on Tuesday. The gathering marked 79 years since the US atomic bombing of the western Japanese city on August 6, 1945, in World War Two.

UN Under-Secretary-General and High Representative for Disarmament Affairs Nakamitsu Izumi read out the message from Guterres at the event.

Guterres wrote, "Nuclear weapons, and the threat of their use, are not confined to history books," but "represent a real and present danger that remains with us today."

He said: "Too many are blind to the fact that we were lucky to end the Cold War without a nuclear war. We cannot press our luck again. Yet some are recklessly rattling the nuclear saber once more."

Guterres said, "We will never forget the lessons" of August 6, 1945. He called for "No more Hiroshimas" and "No more Nagasakis."

The top UN official urged people to create a world without nuclear weapons.