Pope Francis's condition "remains critical" on his tenth day in hospital Sunday, the Vatican said, as Catholics around the world prayed for the 88-year-old's recovery.

Pope Francis's condition "remains critical" but the 88-year-old has not had another respiratory attack, the Vatican said on Sunday, declining again to give a prognosis.

The Argentine pontiff is alert and continues to receive "high-flow oxygen" through a nasal cannula, the Holy See said in its regular early-evening update.

"The Holy Father's condition remains critical; however, since yesterday evening he has not presented any further respiratory crises," it said. 

Francis was given two units of concentrated red blood cells, and his thrombocytopenia "has remained stable," it said.

Thrombocytopenia is a condition that occurs when the platelet count in the blood is too low, which can cause trouble stopping bleeding.

"However, some blood tests demonstrate an initial, mild, renal failure, currently under control," the statement continued. 

It added, "The complexity of the clinical picture, and the need to wait for the pharmacological treatments to have some feedback, mean that the prognosis remains reserved."

The Vatican added that the pope took part in mass on Sunday morning, in his papal suite on the 10th floor of Rome's Gemelli hospital, alongside those looking after him.

Francis, who has been head of the worldwide Catholic Church since 2013, was admitted on 14 February.

He was initially diagnosed with bronchitis but it developed into double pneumonia, and on Saturday the pope suffered a prolonged respiratory attack.