Pope Francis on Sunday told Catholics in China to be "good Christians and good citizens", using his visit to Mongolia to help ease tensions between the Vatican and Beijing.

Following a mass before the scant Catholic population in Mongolia's capital of Ulaanbaatar, Francis turned his attention to officially-atheist China, some of whose citizens had flown in for the pope's visit.

Flanked by the bishop and bishop emeritus of Hong Kong, the 86-year-old pope said they joined with him to send "a warm greeting to the noble Chinese people".