Pope Francis' 2nd-7th July trip to the Democratic Republic of Congo and South Sudan has been indefinitely postponed because of the 85-year-old pontiff's knee ailment, the Vatican said on Friday.

A statement said the postponement was made 'with regret' at the requests of the pope's doctors, who have been treating him for a torn ligament in his knee. The condition has forced him to use a wheelchair for the last month or so.

The decision was taken "in order not to jeopardize the results of the therapy that he is undergoing for his knee," the statement by spokesman Matteo Bruni said.

The announcement was made just a few days after the Vatican had released the names of journalists whose requests to accompany the pope on the papal plane had been accepted and published and preparations in both countries were well under way.

The Vatican did not say if the pope's scheduled 24th-30th July trip to Canada would be affected by his knee ailment.

Despite his ailment, the pope has been keeping up a regular schedule of audiences. He had four audiences on Friday, including one with European Commission President Ursula von der Leyen.

Previous pope have had to cancel trips relatively last in the planning for either health or security reasons in the countries they were due to visit, particularly Pope John Paul, who was in bad health in the final years of his 27-year pontificate.