Pope Francis, who is battling pneumonia and a complex lung infection, remains in a critical condition, the Vatican has said.

The pontiff, 88, was admitted to Rome’s Gemelli hospital on 14 February and was subsequently diagnosed with a respiratory tract infection and pneumonia in both lungs.

Francis was given supplemental oxygen and blood transfusions on Saturday after a prolonged asthma-style attack and required blood transfusions for a low platelet count, doctors said.

“The condition of the Holy Father remains critical; however, since last night he has not experienced further respiratory crises,” the Vatican said on Sunday.

Blood tests had also indicated a “mild renal insufficiency, which is currently under control”, the statement added.

“The complexity of the clinical picture, and the necessary wait for the pharmacological therapies to show some effect, require that the prognosis remains guarded.”

The statement said the pope was receiving “high-flow oxygen therapy” through a nasal cannula, but continued to remain “vigilant and well-orientated”.

On Friday, doctors said the pope’s health remained uncertain and that he was expected to remain in hospital for at least another week.

They warned that the main threat facing the pope would be the onset of sepsis, a serious infection of the blood that can occur as a complication of pneumonia.

Saturday’s blood tests showed that he had developed a low platelet count, a condition known as thrombocytopenia. Platelets are cell-like fragments that circulate in the blood that help form blood clots to stop bleeding or help wounds heal.

Archbishop Rino Fisichella, a senior Vatican official, told participants at a mass in St Peter’s Basilica on Sunday morning they should make their prayers for Francis “stronger and more intense”.

In a message published in lieu of his weekly Sunday Angelus prayer, which he normally delivers from a window overlooking St Peter’s Square, Francis thanked medical staff.

“I am confidently continuing my hospitalisation 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 message that was written in “recent days”.

In New York, Cardinal Timothy Dolan admitted what church leaders in Rome were not saying publicly: that the Catholic faithful were united “at the bedside of a dying father”.

“As our Holy Father Pope Francis is in very, very fragile health, and probably close to death,” Dolan said at St Patrick’s Cathedral, without saying if he had independent information about the pope’s condition.

Francis, who has been head of the Catholic church since 2013, was initially admitted to the Gemelli – which has a special suite for popes – on 14 February with bronchitis.

Doctors first diagnosed the complex viral, bacterial and fungal respiratory tract infection and then the onset of pneumonia in both lungs.

They prescribed “absolute rest” and a combination of cortisone and antibiotics, along with supplemental oxygen when needed.

In its evening update on Saturday, the Vatican said the pope had suffered a difficult day and his “condition continues to be critical, therefore … the pope is not out of danger”.

Dr Sergio Alfieri, the head of medicine and surgery at Gemelli hospital, said: “He knows he’s in danger, and he told us to relay that.”

“The pope gets worse,” Italy’s Corriere della Sera newspaper had said in its Sunday edition, while La Repubblica described it as the “darkest day” at the Vatican.

Francis has said the papacy is a job for life, but has also left the door open to resigning like his predecessor. The late Benedict XVI became the first pope to resign in almost 600 years, citing his advanced age and deteriorating strength.

The Vatican hierarchy tried to tamp down speculation that the pope might decide to resign. There is no provision in canon law for what to do if a pope becomes incapacitated.

Francis has said that he has written a letter of resignation that would be invoked if he were medically incapable of making such a decision.

The Vatican secretary of state, Cardinal Pietro Parolin, gave a rare interview to Corriere della Sera to respond to rumours about a possible resignation after the Vatican issued an unusual and official denial of an Italian media report that said Parolin and the pope’s chief canonist had visited Francis in the hospital in secret.

Given the canonical requirements to make a resignation legitimate, the implications of such a meeting were significant, but the Vatican denied that any such meeting occurred.

Parolin said such speculation seemed “useless” when what really mattered was the health of the pope, his recovery and return to the Vatican.

The 88-year-old pope has maintained a punishing work schedule, despite increasing health issues. Last September, he carried out a mammoth 12-day tour to the Asia Pacific, one of the few places in the world where the Catholic church is growing in terms of baptised faithful and religious vocations.

Associated Press, AFP and Reuters contributed to this report