WASHINGTON (AFP) - US Secretary of State John Kerry warned Russia on Thursday that Washington and Europe were preparing a tough response to a breakaway vote in Ukraine's Crimea, depending on how Moscow reacts.

Russia will respond in some way to Sunday's referendum itself, Mr Kerry told lawmakers, warning: "If there is no sign of any capacity to be able to move forward and resolve this issue, there will be a very serious series of steps on Monday in Europe and here with respect to the options that are available to us."