Pope Francis is expected to call for world-wide nuclear disarmament this week during an Asia tour that will feature visits to the atomic bomb sites in Nagasaki and Hiroshima, Japan.

The danger of nuclear weapons is a topic of special concern to Pope Francis, who said in 2017 that their possession is to be “firmly condemned” because of the risk of accidental detonation.

With that stance, he went further than two of his predecessors, St. John XXIII and St. John Paul II, who called for the abolition of nuclear weapons but taught that their possession could be justified to deter attacks by other powers.

The pope’s weeklong trip began Wednesday in Bangkok, where his agenda includes meetings with Thailand’s prime minister, king and Buddhist supreme patriarch. He travels to Japan on Saturday.

“I pray that the destructive power of nuclear arms is never again unleashed in human history. To use nuclear arms is immoral,” the pope said in a video message to Japan in advance of his trip.