Pope Francis has led his first major public ceremony since health problems forced him to cancel a series of appointments.

Looking tired but relaxed, Francis, 77, appeared to hold up well during the 90-minute Mass in St Peter's Basilica to honour Saints Peter and Paul.

The Vatican has played down the cancellations, attributing them to mild health problem but saying there is no reason to worry about the Pope's health.

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Francis, often chuckling, chatted with 24 archbishops kneeling to each receive a white woollen band, a symbol of shared episcopal power.

He later addressed the faithful in St Peter's Square and appealed to Iraq's leaders to save the nation from war.

As he has done before recently, Francis then asked the public to pray for him, too. "Please don't forget to pray for me," he said. These requests, combined with his recent quip that a pope's job could be unhealthy and the cancellation of appointments in the last few weeks, fuelled concern that Francis was seriously ill.

June 29 is a Roman holiday, and for the occasion, Francis granted an hour-long interview to the city's daily paper Il Messaggero.

He was asked about a suggestion in the Economist magazine that he sounded like a Leninist when he criticised capitalism.

He replied: "I can only say the communists have stolen our flag. The flag of the poor is Christian. Poverty is at the centre of the Gospel."