Francis issued a special prayer for a peaceful end to Venezuela's "grave crisis", which has caused food and medicine shortages.
"The most painful is the humanitarian problem. There are so many people fleeing or suffering," the pontiff told journalists aboard the papal plane on his return trip from a five-day visit to Colombia.
"It's a humanitarian problem that needs to be addressed, and I believe the UN must make itself heard, to help," he said.
On Sunday, Francis issued a special prayer for a peaceful end to Venezuela's "grave crisis", which has caused food and medicine shortages, and clashes at anti-government protests that left 125 people dead from April to July.
The South American country's legislative speaker had called on the pope to plead with President Nicolas Maduro to allow food aid to reach the destitute.
He noted that donations sent to Venezuela in recent months by organisations such as Caritas Chile have not been accepted by the Maduro government.
The Vatican was one of the facilitators of a month-long dialogue between the Venezuelan government and the opposition, which collapsed in mutual recrimination.
"I don't know what Maduro is thinking, I don't know what's going on in his head," Francis admitted to reporters on his plane.