World leaders have accused Russian President Vladimir Putin of a flagrant violation of international law by launching the biggest attack by one state against another in Europe since World War Two.

A statement by Vatican Secretary of State Cardinal Pietro Parolin did not mention Russian leader by name.

"The tragic scenarios that everyone feared are unfortunately becoming reality," said Parolin, who ranks second only to Pope Francis in the Vatican hierarchy.

"But there is still time for good will, there is still space for negotiation, there is still room to exercise a wisdom that prevents partisan interests from prevailing, that protects everyone's legitimate aspirations and spares the world from the folly and horrors of war," he said.

"We believers do not lose hope for a glimmer of conscience in those who hold the destinies of the world in their hands."

Parolin reiterated part of an appeal made on Wednesday by the pope, who warned against "discrediting international law".

Francis has proclaimed Ash Wednesday, which marks the start of the Christian season of Lent and falls on March 2 this year in the Roman Catholic and other Western churches, as an international day of fasting and prayer for peace.

"This appeal takes on dramatic urgency after the start of Russian military operations on Ukrainian territory," Parolin said.

Australian Associated Press