Pope Francis, who has been battling pneumonia and bronchitis for more than three weeks, has thanked the medical staff looking after him for their "tender" care.

For the fourth consecutive Sunday, the Argentine Pontiff was not present for his weekly blessing, but the Vatican shared the text he had prepared.

"While I am here, I think of the many people who are.. close to the sick," he said, describing them as "bringing a little light into the night of pain".

The Vatican said on Saturday the Pope had shown a "good response" in recent days to his treatment at Rome's Gemelli hospital.

"The clinical condition of the Holy Father in recent days has remained stable and, consequently, indicates a good response to the treatment," the Vatican said.

"There is therefore a gradual, slight improvement", it added.

In his blessing, Francis wrote that he had experienced "thoughtfulness of service and the tenderness of care, in particular from the doctors and health care workers, whom I thank from the bottom of my heart".

"I think of the many people who in various ways are close to the sick, and who are for them a sign of the Lord's presence.

"We need this, the 'miracle of tenderness' which accompanies those who are in adversity, bringing a little light into the night of pain."

On Sunday evening, the Vatican said the Pope had joined what were described as "spiritual exercises" for senior Catholic clerics via video link, although the Pope himself was not visible.

The exercises, an annual period of spiritual reflection, were led by a preacher, as usual. The Pope joined as a "recipient", the Vatican said.

On Saturday morning the 88-year-old prayed in the chapel of the papal suite at Gemelli Hospital, the Vatican said.

The Pope has not been seen in public since entering the hospital on 14 February, his longest stretch away from the public eye since his papacy began 12 years ago.

He was admitted to hospital on 14 February after experiencing breathing difficulties - and first treated for bronchitis before being diagnosed with pneumonia in both lungs.

He is particularly vulnerable to pneumonia, an infection of the lungs that can be caused by bacteria, viruses or fungi, after he contracted pleurisy – an inflammation of the lungs – as a young man and had a partial lung removal.

The Pope's doctors believe he is likely to face a long road to recovery, due to his age and medical history.