Pope Francis, who helped broker a historic thaw between the US and Cuba, spoke with Cuban President Raul Castro on Sunday as plans are laid for a trip to both countries in September.

Castro arrived for a nearly hourlong meeting - unusually long for a papal visit - that the Vatican said was strictly private and not a state visit.

On his way back to Cuba from Moscow, Castro stopped in Rome to meet the pope and thank him for the Vatican's mediation, Cuban officials said.