SANTIAGO DE CUBA, Cuba (AFP) - Pope Francis left Cuba on Tuesday for his first-ever visit to the United States, where he may get a slightly chillier reception in some quarters than on the Caribbean island.

The Pope, who played a key role in brokering the recent rapprochement between the Cold War foes, flew out of Cuba's second city Santiago on the same Alitalia plane that brought him from Rome.

Cuban President Raul Castro saw him off at the airport after a four-day visit that featured three cities, three masses, countless handshakes with adoring crowds, and meetings with both Castro and his big brother Fidel, the men who have ruled the communist island since its 1959 revolution.

Francis, 78, will be met at Andrews Air Force Base outside Washington by US President Barack Obama and his wife Michelle, who will also welcome him at the White House Wednesday.

His itinerary also includes landmark speeches to Congress and the UN General Assembly.

Before flying out of Cuba, Francis said mass in Santiago, cradle of the Castros' uprising against dictator Fulgencio Batista, calling for a new kind of "revolution."

Speaking at a basilica to Our Lady of Charity of El Cobre, Cuba's patron saint - a mixed-race Mary that symbolises the island's intertwined Spanish and African roots - he praised her as the embodiment of a "revolution of tenderness."

He urged Cubans to follow her example "to build bridges, to break down walls, to sow seeds of reconciliation," in comments that appeared to allude to the nascent reconciliation across the Florida Straits.

Francis then addressed an audience of families, asking for their prayers as he prepares for a synod on the family next month that has unleashed internal conflicts among the Roman Catholic clergy over sensitive issues such as divorce, homosexuality and unmarried couples.

AN ANTI-AMERICAN POPE?

Francis has received a warm welcome in Cuba, where he is immensely popular for his role in fostering the thaw that saw Washington and Havana restore diplomatic ties in July after more than half a century.

He has had a packed schedule since arriving Saturday afternoon, and at times looked tired in the tropical heat.

But that has done little to dampen the enthusiasm of the fans and faithful who have flocked to see the first Latin American pontiff.

The Argentine pope is broadly popular in the United States, as well - 70 per cent of Americans approve of him, according to one recent poll, compared to 80 per cent of Cubans, in a separate poll.

But for some critics, the dominant themes of his papacy - his critique of consumerism, calls to embrace poverty and condemnation of a "throwaway culture" - sound suspiciously like an indictment of the American way of life.

That was underlined ahead of his trip when Republican Congressman Paul Gosar, who is Catholic, declared he would boycott the pontiff's historic address to Congress to protest his "leftist" views.

The Pope will not have won over such hardline conservatives with his Cuba visit, during which he has discreetly refrained from chastising the communist regime for its crackdowns on dissidents and curbs on civil liberties.

TIGHT SECURITY

Francis is expected to be more provocative in the United States when he addresses Congress on Thursday and the United Nations on Friday.

The Jesuit Pope carefully prepared his speeches for Washington and New York all summer long.

His topics will include critiques of the dominance of finance and technology; a condemnation of world powers over the conflicts gripping the planet; appeals to protect and welcome immigrants; and climate change, including a bold appeal for a radical revolution of the energy industry and a slowdown in growth.

His visit will take place under tight security, with US authorities nervous over the complexities of protecting a pope who insists on traveling in an open vehicle to be close to the masses.

The visit poses a particular security headache in New York, where Francis plans to criss-cross Manhattan at a time when 170 world leaders will be in town for the UN General Assembly.

He will preside over an inter-faith ceremony at Ground Zero in the south, visit a Harlem Catholic school in the north and greet the crowds on a procession through Central Park.

He will wrap up his trip Saturday and Sunday in Philadelphia at an international festival of Catholic families.