Retired Pope Benedict XVI has fallen ill after returning to Vatican City from a trip to Bavaria, German news outlets have reported.
The 93-year-old, who became the first pontiff to resign the position in 600 years in 2013, had been in the southern German region visiting his dying brother.
Newspaper Passauer Neue Presse quoted Peter Seewald, a biographer of Benedict who has published several book-length interviews with the former pontiff, as saying he has been suffering from a facial infection and has been "extremely frail" since his return to Rome.
Benedict, who was born Joseph Ratzinger and elected in 2005, made a four-day visit to the Bavarian city of Regensburg in June to see his 96-year-old sibling, Reverend Georg Ratzinger, who died on July 1.
It was the former pope's first trip outside Italy since resigning.
Mr Seewald claimed to have handed a book to Benedict in person, and told the paper when he met the former pontiff he was optimistic and had been talking about possibly writing himself if his strength picks up.
Benedict, a traditionalist in the Catholic Church, has largely remained out of the public eye since resigning.
He was succeeded by Pope Francis, a reformist who has sought to modernise aspects of the Catholic church.
Agencies have contributed to this report
