Former US Secretary of State Hillary Clinton tried to clarify comments that left the impression she had compared Russian President Vladimir Putin to Nazi dictator Adolf Hitler.
In an appearance at the University of California, Clinton said: "The claims by President Putin and other Russians that they had to go into Crimea ... because they had to protect the Russian minorities.
"That is reminiscent of claims that were made in the 1930s when Germany under the Nazis kept talking about how they had to protect German minorities in Poland and Czechoslovakia".
"I just want everybody to have a little historic perspective - I'm not making a comparison certainly, but I am recommending that we perhaps can learn from this tactic that has been used before".
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.