Pope on ventilation amid battle against double pneumonia.

Pope Francis suffered an "isolated" bronchial spasm which caused him to vomit and led to a "sudden worsening" of his respiratory condition, the Vatican said on Friday evening.

The situation was complicated after the 88-year-old pontiff inhaled some of the vomit, the Vatican said, after which medical staff cleared his airways and placed him on “non-invasive mechanical ventilation” to help him to breathe.

The episode occurred on Friday afternoon at Rome’s Gemelli hospital where the pope has been battling double pneumonia for the past two weeks.

The Vatican statement said that Francis remained "alert and aware at all times" but stressed that the prognosis remains "guarded".

It will take doctors 24 to 48 hours to assess how the bronchospasm would affect the pope's clinical condition, the Vatican said.

The setback comes after health bulletins over the last three days said there had been some "slight improvement" in the pope's medical condition.

On Thursday the Vatican said that pontiff's mild kidney insufficiency detected in recent days had "subsided", while a chest CT scan carried out on Tuesday night revealed a "normal progression" of the lung inflammation.

Last weekend the Vatican said that Francis had suffered a prolonged "respiratory crisis" and had undergone blood transfusions, with tests showing signs of "early, mild" renal insufficiency.

As Francis enters his 15th night in hospital, prayer vigils for his recovery continue outside the Gemelli and in St Peter's Square where a Rosary is being said each night.

Photo credit: Fabrizio Maffei / Shutterstock.com.