The pope's third trip to Asia, and his 32nd trip abroad, takes him to two countries with minority Catholic populations, both evangelised by Jesuit missionaries in the 16th century. Neither have received a Papal visit since the 1980s.

The 82-year-old Francis is particularly fascinated by Japan, where in his youth he hoped to be sent as a missionary before a lung operation cut short those plans.

In Bangkok he will be accompanied by his cousin, Ana Rosa Sivori, a missionary for over 50 years in Thailand, who will also serve as his interpreter.

The pope is to arrive around midday (0500 GMT) Wednesday in Bangkok, ahead of a busy schedule the following day.

Francis expressed his hope to "strengthen the ties of friendship" with Buddhists.

On Thursday, he will meet the Supreme Patriarch of Thailand, Somdej Phra Maha Muneewong, the head of the order of Buddhist monks, at a temple.

Also on the agenda are private meetings with Thai Prime Minister Prayut Chan-o-Cha and King Maha Vajiralongkorn.

He will celebrate mass with members of the Catholic community, which officially accounts for 0.59 percent of the population, or nearly 400,000 faithful.

Friday will be devoted to meetings with priests and bishops of the country, but also a mass in Bangkok dedicated to the youth of the country.

On Saturday, Francis departs for Japan where the following day he visits Nagasaki and Hiroshima, site of the 1945 atomic attacks by the United States that caused 74,000 and 140,000 deaths, respectively, at the end World War II, and hastened the surrender of Japan.

The Pope has previously voiced his fear of nuclear war, and has railed against countries that make money from weapons.

"Together with you, I pray that the destructive power of nuclear weapons will never be unleashed again in human history," said the head of the world's 1.3 billion Catholics in a video message before his departure on Monday. "The use of nuclear weapons is immoral."

Francis is also to meet victims of the devastating 2011 earthquake that struck northeastern Japan and the subsequent tsunami that between them killed 18,500 people and sparked the nuclear power catastrophe at Fukushima.

The most recent visit to the two countries was by the late Pope Jean Paul II, to Japan in 1981 and Thailand in 1984.