By Crispian Balmer

VATICAN CITY (Reuters) -Pope Francis, who has been struggling to overcome double pneumonia for almost three weeks, remained in stable condition throughout the day on Wednesday and has not had any new breathing crises, the Vatican said.

The 88-year-old pope was admitted to Rome's Gemelli hospital on February 14 with a severe respiratory infection that has required continuously evolving treatment.

In a relatively upbeat health bulletin, the Vatican said the pope had carried out some work and had spent much of the day in an armchair. The last time the Vatican specified that the pope had been able to work was on February 27.

However, his doctors reiterated that his prognosis was "guarded", meaning he was not yet out of danger.

The Vatican also said that Francis would once again be reconnected to non-invasive mechanical ventilation overnight after it was removed during the day, underscoring his continued difficulty in shaking off the dangerous illness.

When he is not on mechanical ventilation, which pushes air into the lungs, the pope receives a high-flow of oxygen via a small nasal hose under his nose.

Francis suffered what the Vatican described as two episodes of "acute respiratory insufficiency" on Monday, but has had no repeat of this since then.

For the first time since February 24, the Vatican said the pope had made a call to the Catholic parish in Gaza, which he has done frequently during the Israel-Hamas war.

The pope has not been seen in public since entering hospital, his longest such absence since his papacy started 12 years ago. His doctors have not said how long the treatment might last.

Francis' prolonged absence has stirred speculation, even from senior cardinals, that he could follow in the footsteps of his predecessor Benedict XVI and resign.

But biographers and friends of the pontiff described him as a "fighter," with no plans to resign.

(Reporting by Crispian Balmer; additional reporting by Cristiano Corvino and Giulia Segreti; editing by Bernadette Baum and Mark Heinrich)