Foreign Secretary William Hague says he believes Russia's intervention in Ukraine will "turn out, over time, to be quite a big miscalculation."
Speaking on The Andrew Marr Show, he added he wanted to see diplomatic progress in Ukraine with tensions reduced.
Barack Obama warned Russia that the West will be forced to apply a cost to Moscow if it fails to change course in its dispute with Ukraine.
The ballot paper that the people of Crimea will use in Sunday's referendum was published today
Vitali Klitschko's visit to the eastern city of Donetsk is clearly an attempt to try to ease the divisions with Ukraine.