“We mourn those killed, whose memory will never fade. We remember the terrible destruction wrought upon this city and Hiroshima. We honour the unrelenting strength and resilience of the people of Nagasaki to rebuild,” he said.

New arms race

Yet despite the terrible lessons of 1945, humanity is now facing a new arms race as nuclear weapons are being used as tools of coercion, he noted.

He said weapons systems are being upgraded, and placed at the centre of national security strategies, making these devices of death faster, more accurate, and stealthier at a time of division and mistrust among countries and regions.

“The risk of nuclear catastrophe is now at its highest level since the Cold War,” he warned.

“In the face of these threats, the global community must speak as one. Any use of nuclear weapons is unacceptable. We will not sit idly by as nuclear-armed States race to create even more dangerous weapons.”

Strengthening disarmament efforts

Mr. Guterres stressed that disarmament is at the heart of his Policy Brief on a New Agenda for Peace, launched last month. It calls on Member States to urgently recommit to pursuing a world free of nuclear weapons, and to reinforce the global norms against their use and proliferation.

“Pending their total elimination, States possessing nuclear weapons must commit to never use them. The only way to eliminate the nuclear risk is to eliminate nuclear weapons,” he said.

The Secretary-General added that the UN will continue working with world leaders to strengthen the global efforts towards disarmament and non-proliferation, including through the Treaty on Non-Proliferation of Nuclear Weapons (NPT) and the Treaty on the Prohibition of Nuclear Weapons.

NPT talks have been taking place at the UN in Vienna this month and will conclude on Friday. The treaty entered into force in 1970 and aims to prevent the spread of nuclear weapons and further the goal of nuclear disarmament.

Tribute to survivors

Mr. Guterres also paid tribute to the survivors of the atomic bombings, known as hibakusha. He said their powerful and harrowing testimonies will forever serve as a reminder of the need to achieve a world free of nuclear weapons.

“I have pledged to do everything in my power to ensure that the voices and testimonies of the hibakusha continue to be heard,” he said.

He called on young people - the world's future leaders and decision makers - “to carry their torch forward”, saying “we can never forget what happened here. We must lift the shadow of nuclear annihilation, once and for all.”