UN chief Ban Ki-moon, speaking in Japan ahead of ceremonies marking the 1945 atomic bombing of Hiroshima and Nagasaki, said on Tuesday the world must strive to become free of nuclear weapons.

Ban will this week become the first UN secretary-general to attend the Peace Memorial Ceremony in Hiroshima and the first to visit Nagasaki, both of which were hit by nuclear bombs in the closing days of World War II.

He began his tour of Japan on Tuesday in Tokyo, where he met Foreign Minister Katsuya Okada and is expected to meet Prime Minister Naoto Kan on Wednesday before heading to the two cities in western Japan.

In both Hiroshima and Nagasaki, Ban, a South Korean national, will visit memorials to Korean victims of the atomic bombs, as well as memorials to the Japanese victims.

“I hope through my attendance in the ceremony I’ll be able to send… a strong message to the whole world that we must strive and work harder to realise a world free of nuclear weapons,” Ban said after meeting Okada.

“We must help them realise their aspiration to see the world free of nuclear-free weapons,” the UN chief said, referring to the survivors’ hope of nuclear disarmament.

“We must help those hibakusha (atomic-bomb survivors), whose lives may be just a matter of just a few years, who are getting older and older,” he said.

Japan, the only nation to have been attacked with atomic bombs, has long campaigned for nuclear disarmament, even though it relies on the US nuclear umbrella for its defence.