Pope Francis, who has spent the last two nights in hospital, has apologised for missing a mass at the Vatican on Sunday, saying that he was still being treated for bronchitis and thanking his doctors for looking after him.

Francis, 88, was admitted to a Rome hospital on Friday with a respiratory tract infection. His doctors have ordered him to rest and he was unable to deliver his regular Sunday prayer to pilgrims in St Peter's Square.

"I would have liked to be among you but, as you know, I am here at the Gemelli Hospital because I still need some treatment for my bronchitis," Francis said in a short written version of the prayer.

"Thank you for the affection, prayer and closeness with which you are accompanying me in these days, and I would like to thank the doctors and healthcare workers in this hospital for their care," he added.

The cardinal who led mass on Sunday in the absence of Francis opened the service with a message of support.