Pope Francis is considering adding a stop in Cuba to his US trip in September but no decision has been made yet, the Vatican said.

Pope Francis has been credited with having helped the US and Cuba reach their historic rapprochement by writing to the leaders of both countries and having the Vatican host their delegations for the final negotiations.

Vatican spokesman the Rev Federico Lombardi said Pope Francis "is considering the idea of a Cuba leg" but that discussions with Cuba are at an early stage. He said it is too early to say that a decision has been taken or that there is an operational plan under way.

Pope Francis is scheduled to visit three US cities in the last week of September. He will address Congress and meet with President Barack Obama at the White House, address the UN in New York and attend a church rally for families in Philadelphia.

If a Cuba stop is confirmed on either end of the US trip, Pope Francis would become the third pope to visit the island nation after the historic 1998 visit of Pope John Paul II during which he said Cuba should "open itself up to the world, and may the world open itself up to Cuba".

Emeritus Pope Benedict XVI followed up with a 2012 trip during which he voiced the Vatican's long-standing position that the US embargo was unjust and only hurt the most vulnerable on the island.

Pope Francis also has spoken out against the US embargo while also condemning socialism.

Pope Francis' personal intervention in the US-Cuban thaw was one of the most tangible signs that he wants the Vatican to be a greater player in international diplomacy. A more controversial intervention was his recent declaration that the slaughter of Armenians by Ottoman Turks a century ago was "genocide".