Pope Francis has embarked on the longest and farthest trip of his tenure, which will see him fly nearly 33,000km (20,500 miles) to Indonesia, Papua New Guinea, Timor-Leste and Singapore.

It's a particularly challenging journey for a man who turns 88 in December and has been battling a spate of health issues.

Francis is expected to highlight environmental concerns and the importance of interfaith dialogue during his 11-day trip. Timor-Leste is the only one of the four countries that is predominantly Catholic.

He is expected to arrive in Jakarta at around 11:30 local time (04:30 GMT) on Tuesday, where he will visit the Indonesian capital's main mosque.

Parts of Francis's trip, which was originally scheduled in 2020 but postponed due to the pandemic, will retrace the steps of St John Paul II, who also visited the four nations during his 27-year pontificate.

"Today I begin an Apostolic Journey to several countries in Asia and Oceania," Francis wrote on X on Monday. "Please pray that this journey may bear fruit."

Since his election in 2013, Francis has urged the Catholic Church to bring God's comfort "toward the periphery" - referring to communities who are marginalised or far away.

He is only the third pope to visit Indonesia, which has the largest population of Muslims globally.

During his four days there, he will meet with outgoing president Joko Widodo and hold a mass for some 70,000 people, according to the Vatican News.

Nasaruddin Umar, the grand imam at the Jakarta mosque Francis will be visiting, told the AP he hopes the visit will offer opportunities to "discuss the common ground between religious communities and emphasise the commonalities between religions, ethnicities, and beliefs".

In Papua New Guinea, Francis will travel to the remote, poverty-wracked city of Vanimo to meet with missionaries from his native Argentina who have been reaching out to tribal communities.

Miguel de la Calle, an Argentine missionary in Papua New Guinea's north-westernmost city, said he hoped the Pope's visit would "significantly boost" ongoing evangelisation efforts in the territory.

People have been travelling from all Papua New Guinea and even across the border from Indonesia to see the Pope, he told Vatican News.

"Some have been walking for days due to the scarcity of transportation," Father Miguel said.

In Timor-Leste, Francis will officiate mass in the capital Dili, on the same seaside esplanade where John Paul II spoke in 1989 to comfort local Catholics who suffered under Indonesia's occupation of the territory. Timor-Leste gained independence in 2002.

The sheen of Francis's visit to the country has dimmed in recent days, however, following revelations that hundreds of homes in the area were bulldozed. Nearly 90 residents were told to find somewhere new to live before he arrives.

Francis will wrap up his trip in Singapore, where three-quarters of the population are ethnically Chinese and Mandarin is one of four official languages. This visit is widely seen as an attempt to improve ties with China.

No pope has been able to visit China to date, as relations between the Vatican and the Chinese Communist Party have been strained by disputes over who can appoint bishops in the country.

Both sides are believed to have reached a deal on this in 2018, which gives the Vatican a say on such appointments.

During his trip, Francis will be accompanied by a doctor and two nurses. Concerns have been raised over the impact of such an ambitious itinerary on his ailing health.

Francis, who has had part of one lung removed in his younger days, had been struggling with respiratory and mobility problems of late - some of which have led him to miss his weekly Sunday blessings.

In November last year, he cancelled his trip to Dubai for the annual United Nations climate meeting because of a lung inflammation.