Vatican City - Pope Francis' blood tests show a "slight improvement" as he receives hospital treatment for pneumonia, the Vatican said Wednesday, while Italy's prime minister found the 88-year-old joking "as always".

The Holy See said the Argentine pope's clinical conditions were "stable" on his sixth day at Rome's Gemelli hospital, where on Tuesday, it revealed he had been diagnosed with pneumonia in both of his lungs.

"The blood tests, evaluated by the medical staff, show a slight improvement, particularly in inflammatory indices," said the Vatican in a statement Wednesday.

Following breakfast, he "dedicated himself to work activities with his closest collaborators", it added.

World Vatican shares concerning update on Pope Francis' health

Prime Minister Giorgia Meloni also visited the pontiff for 20 minutes, it said, and she found him "alert and responsive".

"We joked as always. He hasn't lost his proverbial sense of humor," the far-right leader said in a statement.

The double pneumonia diagnosis caused widespread alarm over the pope's health after a series of issues in recent years, from colon and hernia surgery to problems walking.

But amid much speculation online, including reports of his supposed death, the Vatican issued an early bulletin Wednesday saying he had spent a "peaceful night" in the hospital's papal suite and had breakfast.

"The pope is breathing on his own. His heart is holding up very well," a source in the Vatican added.

Francis has been speaking to friends by telephone, has been out of bed and sitting in a chair, and working on and off, the source said.