France's foreign minister said on Monday that everything was in place for Russian forces to invade quickly in Ukraine, adding that Europe was ready to impose massive sanctions if it happened.

"If the question is are there elements in pace so that there is a major offensive by Russian forces in Ukraine, then yes it's true. It's possible and quickly," Jean-Yves Le Drian told France 5 TV.

He said the assessment was shared by Paris and its allies, although he said nothing indicated that Russian President Vladimir Putin had made that decision.

"It's blowing hot and cold," he said, suggesting that Russia was leaving the door open for dialog.

Two European diplomatic sources said the United States had told its allies last Friday that one scenario was that Moscow could launch an offensive this Wednesday.

When asked whether sanctions were ready should there by a Russian attack, Le Drian said that was the case and they would be massive even if they would also have a boomerang effect on Europe.

The minister said it was important to pursue dialog to avoid a disastrous conflict that would hurt all sides.