ROME and LONDON -- Pope Francis rested overnight in the hospital, the Vatican said on Sunday.

"The night passed peacefully. The Pope rested," the Holy See said in a short statement.

The 88-year-old pontiff had on Saturday morning suffered a "prolonged" asthmatic respiratory crisis, during which doctors supplied him with oxygen, the Vatican said.

Asthmatic respiratory crisis indicates the pope struggled to maintain a sufficient oxygen level when breathing on his own, according to medical experts.

The Vatican on Saturday said the pope, who has led the Catholic Church since 2013, remained in critical condition.

The pope has been hospitalized at Rome's Gemelli Hospital since Feb. 14 following a bout with bronchitis.

He was diagnosed with pneumonia on Tuesday, according to the Vatican.

ABC News' Ivan Pereira and Youri Benadjaoud contributed to this report.