The United Nations is urging countries armed with nuclear weapons to stick to their no-first-use commitment of the atomic arsenals.

U.N. Secretary-General Antonio Guterres warned Monday that a nuclear arms race has returned amid growing international tension.

Guterres spoke in Tokyo after visiting Hiroshima to commemorate victims of the August 6th, 1945, atomic bombing.

When asked about Russia’s shelling on Europe’s largest nuclear power plant in Ukraine, Guterres said any attack on a nuclear plant is “suicidal.”

Guterres said nuclear disarmament is moving backward, and arms race investments would be better spent fighting climate change and poverty.

He also urged Japan to stop funding coal plants.