Pope Francis will postpone his upcoming trip to the Democratic Republic of Congo and South Sudan due to an ongoing knee problem, the Vatican said Friday. 

“At the request of his doctors, and in order not to jeopardise the results of the therapy that he is undergoing for his knee, the Holy Father has been forced to postpone, with regret, his Apostolic Journey to the Democratic Republic of Congo and to South Sudan,” Vatican spokesman Matteo Bruni said in a statement.

The trip, originally planned for July 2 to 7, will be rescheduled though no new date has been set.

Francis, 85, has been suffering from pain in his right knee in recent weeks and last month relied on a wheelchair for the first time at a public event.  He has cancelled numerous engagements, and postponed a scheduled trip to Lebanon in June, and has sometimes been seen struggling to walk. 

The Vatican has not said officially what the problem is, although sources have told journalists he has chronic arthritis. The pope himself has also spoken of an injured ligament in his knee. 

He told Italian newspaper Corriere della Sera last month he would receive an “intervention with infiltration”, which Vatican sources said involved injecting anti-inflammatories into his joint.

The Vatican, which announced the trip to Africa in March, had already published its schedule. The pontiff was to visit the DRC’s capital of Kinshasa, as well as Goma, the main town in the restive eastern province of North Kivu. 

He was then to head to South Sudan, visiting the capital Juba.

South Sudan, the world’s newest nation, has suffered from chronic instability since independence in 2011, including a brutal five-year civil war. Meanwhile the DRC, which Pope John Paul II visited in August 1985, is struggling to contain dozens of armed groups in the east of the vast nation.