Pope Francis today made a "heartfelt" appeal to resolve the Ukraine crisis through dialogue and co-operation.

Addressing huge crowds gathered in rainy Vatican, the Pope asked the faithful to pray for Ukraine.

"I ask you to keep praying for Ukraine, which is living through a very delicate situation, while I hope that all the different parts of the country work together to overcome misunderstandings and together work towards building a future for the country. I extend a heartfelt appeal to the international community to support any initiatives in favour of dialogue and peace," Pope Francis said.

Ukraine mobilised for war today and called up its reserves, after Russian President Vladimir Putin threatened to invade the country in the biggest confrontation between Moscow and the West since the Cold War.

Pope Francis skipped a planned visit to a Rome seminary on Friday evening because he was suffering from a slight fever and was advised to rest, the Vatican said.

It was the first time since his election nearly a year ago that the pope, 77, has cancelled a public engagement.