US President Barack Obama warned Russia that the US and the international community were "not recognising what is happening in Crimea.
Speaking to world leaders, he rejected "the notion that a referendum sloppily organised over the course of two weeks" would "somehow be a valid process".
He appealed for other leaders to put pressure on Moscow to get Mr Putin to "understand there is a choice to be made here" and said:
"We also are concerned about further encroachment by Russia into Ukraine".
An unusually frank Nato Secretary General issued an unequivocal warning on the course of events should Russia attack Latvia or Estonia.
The West responds with sanctions and closer political ties with Ukraine as Russia's President seals the annexing of Crimea to much fanfare.
Supporters of the controversial fracking industry believe shale gas could be America's most potent weapon in the stand-off with Russia.