Pope Francis's condition "remains critical," the Vatican said Sunday on his 10th day in hospital as Catholics around the world prayed for the 88-year-old's recovery.

Francis has not had another respiratory attack since suffering a prolonged episode on Saturday morning, but continues to receive "high-flow" oxygen through a nasal cannula, the Holy See said in an update.

The Argentine pontiff was during the day given two units of concentrated red blood cells and his thrombocytopenia — a condition that occurs when the platelet count in the blood is too low — is stable, it said.

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

Francis is alert but "the complexity of the clinical picture, and the need to wait for the pharmacological treatments to have some effect, mean that the prognosis remains reserved," it concluded.

The Pope, the leader of the worldwide Catholic Church since 2013, was admitted to Rome's Gemelli hospital on 14 February with breathing difficulties.

He was initially diagnosed with bronchitis but this developed into pneumonia in both lungs — and on Saturday night, the Vatican warned for the first time that his condition was critical.

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

Pray for me

In a message published Sunday but written in the past few days, Francis thanked hospital staff and said he had confidence in his treatment.

"I am confidently continuing my hospitalization at the Gemelli Hospital, carrying on with the necessary treatment — and rest is also part of the therapy!" he said.

"I ask you to pray for me," he concluded in the text published instead of his weekly Sunday Angelus, which he normally delivers from a window overlooking St. Peter's Square.

Well-wishers have been leaving candles outside the Gemelli, where Francis is staying in a special papal suite on the 10th floor, and where the Vatican said he took part in a mass on Sunday morning.

"I am praying for him, for his health, because he is a special person for all of us," Italian teacher Ilde Zito told AFP at the hospital.

Prayers and messages of solidarity also came in from among the almost 1.4 billion Catholics across the globe, alongside other Christians and world leaders.

In the cathedral of Buenos Aires, where Francis was once archbishop, TV screens at the entrance showed images of then Jorge Bergoglio's good works, and the priest held a special mass on Sunday.

"He is strong, he has always been strong, but there is nature," Hector Armando Diaz, 76, a retired sales clerk, told AFP there.

"I know he is very ill and old. I hope he recovers, but it makes me sad."

In Iraq's northern city of Mosul, a former bastion of the Islamic State group that Francis visited in 2021, at least a dozen churches also held prayers for him.

"This is the least we can do to express our solidarity, love, and appreciation for this great person," said university professor Adnan Hadi.

Mexico's President Claudia Sheinbaum offered her hopes on social media for a speedy recovery for the "great humanist" Francis, the first pope from the Americas.

Suffering more

In its evening update Saturday, the Vatican had also said Francis was alert and "spent the day in an armchair, even if he was suffering more than" the day before.

But the news of his respiratory attack and blood transfusions sparked widespread shock, and alarm.

"The Pope gets worse," headlined Italy's Corriere della Sera newspaper Sunday. La Repubblica described it as the "darkest day" at the Vatican.

Francis's continued hospitalization has also sparked speculation about whether he might step down.

He has always left the door open to following his predecessor, Benedict XVI, who in 2013 became the first pope since the Middle Ages to resign.

But he has repeatedly said it was not the time.

The pope maintains a punishing work schedule. He carried out a mammoth 12-day tour to the Asia-Pacific in September but has suffered increasing health issues.

He underwent colon surgery in 2021 and an operation for a hernia two years later. He is overweight and has constant hip and knee pain, which forces him to use a wheelchair.