Pope Francis told Ukrainian Prime Minister Arseniy Yatsenyuk that he would "do everything possible" for the country, amid fears that Russia could be about to invade.
Yatsenyuk said he asked Francis "to pray for Ukraine and for stability in Europe" and told him he was grateful for the support.
He said the Vatican had already averted wars during the Cuban Missile Crisis in 1962 and the Argentina-Chile territorial dispute in 1978.
The Vatican said in a statement that Francis and Yatsenyuk had discussed the "specific role" that religious organisations could play "in fostering mutual respect and harmony".
"Mention was made of possible further initiatives by the international community in this regard," the statement said.
Yatsenyuk's spokeswoman said the premier was cutting short his trip to Rome and will miss the canonisation of John Paul II and John XXIII, as tensions mount in the eastern part of the ex-Soviet country and Western nations threaten sanctions.
Yatsenyuk spent 18 minutes behind closed doors with the pope, who had urged the international community to "prevent violence" in Ukraine in his Easter Sunday message.
At an exchange of gifts, Yatsenyuk presented Francis with a photograph of Maidan square in Kiev on New Year's night.
"This is where Ukrainians fought for their freedom and rights. Millions of people," he said.
The pope in return offered the Ukraine leader a pen, saying "I hope this pen will sign the peace", to which Yatsenyuk replied "I hope so."