Cuban President Raul Castro arrived at the Vatican on Sunday to thank Pope Francis for his role in brokering the rapprochement between Havana and Washington.
The first South American pope played a key role in secret negotiations between the United States and Cuba that led to the surprise announcement in December that they would seek to restore diplomatic ties after more than 50 years of tensions.
Castro, who was accompanied by his Foreign Minister Bruno Rodriguez Parrilla, was due to hold a "strictly private" meeting with the pontiff in a small room adjoining the Paul VI Audience Hall, where large gatherings are held in the Vatican.
Pope Francis arrived ten minutes ahead of Castro, while a dozen unformed Swiss Guards stood to attention in front of the building when the limousine bearing the Cuban flag arrived.
The Holy See has said the Argentine pope personally mediated between the two sides, and the Vatican hosted delegations from the two countries in October.
US theologian Miguel Diaz, a former ambassador to the Holy See, said Francis would reprise the words of Polish pope John Paul II, who made a historic first papal visit to Cuba in 1998.
"Let Cuba open itself to the world, and let the world open itself to Cuba," John Paul II urged during the visit, when he was accompanied by Jorge Bergoglio, then auxiliary bishop of Buenos Aires and today Pope Francis.
The pope will "certainly reiterate" John Paul II's urging "now that Cuba is trying to step up its involvement in the economic world and international relations," Diaz told the Italian news agency Adnkronos.
Castro's stop at the Holy See, announced only Tuesday, follows a visit to Russia, where the Cuban leader attended a grandiose World War II victory parade on Saturday.
He will meet Italian Prime Minister Matteo Renzi in Rome later on Sunday.
The Vatican announced last month that Pope Francis would visit Cuba in September, becoming only the third pontiff to do so after John Paul II in 1998 and Benedict XVI in 2012.
Francis will go on from Cuba to the United States and a meeting with President Barack Obama.
Castro's ailing older brother Fidel came to the Vatican in 1996 when he met pope John Paul II.
The Catholic Church has consistently backed calls for the lifting of the US trade embargo against Cuba, while staunchly supporting Cuban Catholics, pressuring Havana to release political prisoners, many of them Catholic activists.
The Vatican also kept its distance from Cuban exiles based in Miami, Florida, clamouring for Havana's Marxist regime to be overthrown by force.
When the now retired Benedict XVI visited Cuba in 2012 he had long and warm talks with Fidel Castro, who is now 88.
The Vatican's mediation between Cuba and the US administration was a major success for the Holy See and had a considerable impact in mainly Catholic Latin America.
Other diplomatic efforts have been less successful, including a bid to help resolve the political crisis in Venezuela and a longstanding drive to encourage reconciliation between the Colombian government and guerrilla movements there.
The Holy See successfully mediated in 1984 between Chile and Argentina in their dispute over the Beagle Channel at the extreme southern tip of South America.