UN Secretary-General Antonio Guterres says Japan's experience as a victim of atomic bombings gives it "a particular moral authority" to promote nuclear disarmament and non-proliferation.
Guterres discussed the matter on Tuesday, ahead of his visit to the Japanese city of Hiroshima to attend events related to the Group of Seven summit.
Nuclear disarmament and non-proliferation are expected to be high on the agenda at the summit, which opens Friday.
Guterres called on G7 leaders to issue a strong message, amid the heightened nuclear threat in Ukraine, where Russia's invasion is grinding on through a second year.
He stressed the need for "revitalizing" nuclear disarmament, and said it is time for nuclear powers to affirm that they will not use the weapons in any circumstances.
The UN chief noted the significance of holding the summit in Hiroshima, which was devastated by an atomic bomb near the end of World War Two.
Guterres also plans to attend a session to discuss measures against climate change. He said G7 leaders have particular responsibilities as representatives of the richest countries in the world.
He called not only for additional efforts to dramatically reduce greenhouse gas emissions, but also financial and technical support for emerging economies.