Pope Francis says he wants to go to Mongolia in September

Pope Francis returns from his visit to Democratic Republic of Congo and South Sudan

By Philip Pullella

ABOARD THE PAPAL PLANE (Reuters) - Pope Francis said on Sunday that he expected to continue travelling despite his knee ailment, joking that "weeds never die" and adding that he hoped to go to Mongolia in September.

The trip would be the first ever by a pope to the Asian country.

During his customary conversation with reporters on his return from a foreign trip, this time from Democratic Republic of Congo and South Sudan, the 86-year-old pope was asked about his health and travel plans.

For months, a chronic knee ailment has forced him to use a wheelchair and a cane. During recent trips, he has held the news conference on the plane while sitting, as he did on Sunday.

"On September 29 I will go to Marseille and from there I will fly to Mongolia but that is not yet certain, there is a possibility," he said.

The day trip to the southern French city of Marseille was known about but the mention of Mongolia was a complete surprise to reporters, so much so that some thought he might have said Angola.

Speaking of his health, Francis, who became pope nearly 10 years ago, said "You know that weeds never die. It (his health) is not like it was at the start of the pontificate, that is true. This knee is bothersome but it moves ahead slowly. Then we will see."

Last August Francis named Archbishop Giorgio Marengo, an Italian, the first cardinal to be based in Mongolia, where he is the Catholic Church's administrator.

The country has fewer than 1,500 Catholics but is strategically significant because it borders with China, where the Vatican is trying to improve the situation of Catholics in the communist country.

Francis, who was on his way home from his 40th foreign trip as pope, also said he would probably go to India next year. He is due to go to Portugal this August.

(Reporting by Philip Pullella; Editing by Frances Kerry)