Vatican City - Pope Francis's condition was continuing to improve, the Vatican said Thursday, as the 88-year-old pontiff marked two weeks in the hospital with pneumonia in both lungs.

"The clinical conditions of the Holy Father are confirmed to be improving today also," said the Vatican in an evening bulletin on the pope's health.

It was the second consecutive day the Vatican cited an improvement in the critical condition of the Argentine pope, who was admitted to Rome's Gemelli hospital on February 14.

As pilgrims and other well-wishers continue to gather to pray and leave offerings outside the hospital, medical experts – together with the Vatican – have warned that Francis is not out of danger.

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"Given the complexity of the clinical picture, further days of clinical stability are necessary to resolve the prognosis," the Vatican statement said.

After first being admitted to hospital with breathing difficulties, the Vatican revealed that the pope – who had part of a lung removed as a young man – had double pneumonia.

But on Wednesday, it said that Francis's clinical conditions had shown a "slight improvement" while cautioning that his prognosis remained "reserved".

A respiratory attack Francis suffered over the weekend that required high levels of oxygen has not been repeated, although he continues to receive oxygen and do breathing exercises.

On Thursday, he "alternated high-flow oxygen therapy with a venturi mask," the Vatican said, referring to an oxygen mask that delivers oxygen at controlled concentrations.

"In the morning, the Holy Father dedicated his time to respiratory physiotherapy, alternating it with rest," said the Vatican.

In the afternoon, following another session of therapy, he prayed in the chapel located on the 10th floor of the Gemelli in the special papal suite, said the Vatican.

He then spent time working, it said.

The pope's latest improvement comes after his deteriorating condition at the weekend sparked fears that the head of the world's nearly 1.4 billion Catholics might not pull through.