Pope Francis has described life imprisonment as being like a "hidden death penalty" as he began his first visit to the United States.
Ahead of becoming the first pope to address Congress on Thursday, he remained tight-lipped on the contents of the speech, amid much speculation about what he might say about the US trade embargo on Cuba.
Speaking to reporters on his flight from Cuba, he expressed distress at long prison sentences, saying: "Life imprisonment is almost a hidden death penalty. You are there and you are dying every day without any hope of freedom."
On the subject of his speech, he said it would address "bi-lateral relations and multi-national relations as a sign of progress and coexistence".
Discussing US relations with Cuba, he said: "My desire is that they end up with a good result, that they reach an accord that satisfies both sides, an accord, certainly."
On Friday, the US announced new rules that further eased trade with the Communist-run island.
Pope Francis' first US visit has sparked an unexpected craze among Americans - dressing dogs up in miniature versions of his papal garb.
A five-year-old girl managed to breach the Pope's security to deliver him an important letter.
Pope Francis is due to arrive in the US, but he will face a political test on Thursday when he addresses Congress.