Pope Francis is still "slightly unwell" and has postponed all official audiences for Friday, but is working from his residence.

In a statement, the Vatican did not specify what the 83-year-old pope was suffering from.

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At his general audience on Wednesday, he appeared to have a cold and spoke in a slightly hoarse voice, and he coughed during an afternoon Ash Wednesday service in a Rome church, his last appearance outside the Vatican.

Spokesman Matteo Bruni said the pope had said early morning mass as customary, in the chapel of the Santa Marta guest house where he lives, and greeted those who attended.

"But he thought it was best to postpone today's official audiences. The meetings scheduled to take place in Santa Marta will take place regularly," Bruni said.

On Friday morning, he was to have received executives from Microsoft, IBM, and other technology companies.

His speech to them from the Apostolic Palace was to have been streamed to participants of a conference in Rome on ethics in Artificial Intelligence.

At the conference, Microsoft, IBM, the Vatican and the UN Food and Agriculture Organisation (FAO), signed a memo of understanding on AI.

The pope usually spends mornings making speeches to groups and meeting heads of state in the Vatican's Apostolic Palace.

Unlike previous popes, he chose not to live in its spacious papal apartments and opted for simple quarters in Santa Marta, where he spends the rest of the day mostly working on Church matters.

On Thursday, he cancelled a visit to a Rome basilica.

Pope Francis is missing a part of one lung. It was removed when he was in his early 20s in his native Buenos Aires after he suffered from tuberculosis.

He also suffers some leg pain due to sciatica, for which he undergoes regular physical therapy and which explains his occasional difficulty climbing steps. But he is in generally good health.