French Foreign Minister Laurent Fabius said on Tuesday that sanctions against Russia over Ukraine could be imposed as early as this week if Moscow failed to respond positively to a proposition designed to calm the crisis.

Fabius told radio station France Inter that a referendum in the Crimea region on joining Russia set for March 16 was illegitimate and that the annexation of the region by Russia would be illegal.

"We cannot accept something that is illegal and which will also have very serious consequences," Fabius said.

The minister said the "only legitimate vote" was that for the president of Ukraine on May 25.

Fabius said U.S. Secretary of State John Kerry had sent a proposition to Moscow in an effort to calm the situation.

"If they respond positively, John Kerry will go to Moscow, and in that case the sanctions won't be immediate," Fabius said. "If they don't respond, or if they respond negatively, in that case there are a series of sanctions that can be taken as early as this week."

Fabius said these could include freezing the assets of individual Russians or Ukrainians, as well as sanctions relating to travel visas.

A pro-Russian force opened fire and seized a Ukrainian military base in Crimea on Monday, and NATO announced reconnaissance flights along its eastern frontiers as confrontation around the Black Sea peninsula showed no sign of easing.