Ukrainian President Volodymyr Zelenskiy asked Pope Francis on Saturday for help to win the release of prisoners of war held by Russia and Russian-backed separatists.
"(The pope) does everything possible to achieve peace and harmony throughout the world," Zelenskiy said in a tweet after their meeting at the Vatican.
"I asked for help with the release of Ukrainians captured in Donbass, Crimea and Russia," he said.
Relations between Ukraine and Russia collapsed following Moscow's annexation of the Crimean peninsula in 2014 and its subsequent support for separatists in a conflict in eastern Ukraine.
The Vatican has diplomatic relations with both Ukraine and Russia.
Ukrainian government forces and pro-Russian separatists in the east completed a large-scale prisoner swap on December 29 in the breakaway Donbass region. It is not clear how many Ukrainian government soldiers are still being held, but activists say there are about 100.
Zelenskiy won a landslide election victory in April, promising to end the five-year-old conflict and bring prisoners home.
The 42-year-old president, a former comedian, said he emerged from the meeting with the pope, who is nearly twice his age, "inspired by our talk about peace in Ukraine".
During the photo session of the meeting, which was open to reporters, Francis gave Zelenskiy a medal of St. Martin of Tours and said he hoped the saint "will protect your people from war".
A Vatican statement made no mention of what the pope and Zelenskiy spoke about in their private talks.
It said the president did discuss the conflict and its effects on the civilian population in separate talks later with the Vatican's Secretary of State, Cardinal Pietro Parolin and its foreign minister, Archbishop Paul Gallagher.