VATICAN CITY (AFP) - Pope Francis told Ukraine Prime Minister Arseniy Yatsenyuk on Saturday that he will "do everything possible" for peace in the country, amid fears that Russia could be about to invade.

The Ukrainian Premier is 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 former Soviet country and Western nations threaten sanctions.

Mr 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, Mr Yatsenyuk presented Pope 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 gave the Ukraine leader a pen, saying "I hope this pen will sign the peace", to which Mr Yatsenyuk replied: "I hope so."

As the two leaders parted, Pope Francis put his hand on his chest and said "I will do everything possible" for peace.

The Group of Seven rich countries have agreed to slap new sanctions on Moscow as early as Monday amid fears in the West that Russia could be about to invade.

Nerves frayed after pro-Russian rebels holding a group of international observers in the area earlier on Saturday accused them of being "Nato spies" and vowed to continue detaining them.