Cuban President Raul Castro said on Sunday that Pope Francis' wisdom and modesty was making him think about returning to the Catholic faith.

His comments were made after a meeting between the Communist leader and the Pope at the Vatican yesterday, during which he thanked the Pope for the part he played the restoration of diplomatic relations between the US and Cuba, which was announced in December.

Francis wrote personal letters to both leaders and allowed the Vatican to be used as a location for secret negotiations.

"I am very happy. I have come here to thank him for what he has done to begin solving the problems of the United States and Cuba," Castro said.

The President said he read all of the Pope's speeches and commentaries. "If he continues in this vein I will start praying again and will return to the Catholic Church. I'm serious," he said.

He also pointed out that like Francis he had also attended Jesuit-run schools. "The pontiff is a Jesuit, and I, in some way, am too," Castro said.

Castro stopped off at the Vatican on the way home from attending the Russian World War II victory celebrations in Moscow. The Vatican said the meeting was strictly private and not an official state visit.

Francis will make a two-day visit to Cuba on his way to the US in September, at the invitation of both the Cuban government and the Cuban Roman Catholic Church.

The President gave the Pontiff a painting of a cross by a Cuban artist, inspired by the plight of migrants trying to cross the Mediterranean – a subject that the Pope has spoken on a number of times.

In return, the Pope gave Castro a medal of St Martin of Tours, a French saint known for his care of the poor. He also gave him a copy of his Apostolic Exhortation Evangelii Gaudium (The Joy of the Gospel).