Pope Francis was due to set off on Tuesday evening for his first trip to Thailand and Japan.
After his three-day visit to Thailand, he is due to deliver a message against the use of nuclear weapons in the Japanese cities of Nagasaki and Hiroshima.
The U.S. dropped atomic bombs on the two cities during World War II, razing them and killing tens of thousands of people.
In Japan, the pontiff is also expected to meet survivors of the 2011 Fukushima nuclear power plant disaster that followed a powerful earthquake and tsunami.
Francis is only the second pope, after John Paul II, to visit the two Asian countries.
In Thailand, a meeting between the pope and King Maha Vajiralongkorn in Bangkok is expected to take centre stage.
The king, who also maintains a villa on Lake Starnberg near the southern German city of Munich, has recently been in the headlines for his former concubine, who was suddenly removed from her “office” and has since disappeared without a trace.
It is unclear whether Francis will comment on the controversial subjects of sex tourism and trafficking in Thailand.
Catholics are a tiny minority in both Buddhist countries.
Francis is scheduled to return to Rome on Nov. 26.