Pope Francis has told South American bishops to speak "courageously'' at a meeting on the Amazon, where the shortage of priests is so acute that the Vatican is considering ordaining married men and giving women official church ministries.

Francis on Monday opened the work of the three-week synod, or meeting of bishops, called to chart new ways to better minister to indigenous communities and care for the rainforest they call home.

Among the most contentious proposals on the synod agenda is whether married community elders could be ordained priests. Currently indigenous Catholics in remote parts of the Amazon can go months without seeing a priest or receiving the sacraments.

Another proposal calls for bishops to identify new "official ministries'' for women, though priestly ordination for them is off the table.