US Secretary of State John Kerry has warned Russia that ratifying a Crimea referendum result in the Russian parliament will amount to a "back-door annexation" of the Ukrainian region.
Mr Kerry said the international calls for sanctions as "a response" to the "illegal" referendum being held were not a threat to Russia but a necessary step to "respecting the integrity of Ukraine".
"It is not our preference," he said, but a "justified" step due to the "breach of international law".
He added, though, that the international community still "does not know definitively" what Vladimir Putin will decide about Russia's response to a referendum on Crimea and insisted the US had put "plenty of options on the table" for the Russian President to take a different course.
John Kerry warned of "consequences" after his Russian counterpart made clear Moscow would respect the outcome of Crimea's referendum.
Foreign ministers meeting in Brussels will decide on Monday on sanctions for Russia if there is no de-escalation in Ukraine.
Crimea's new information minister has told ITV News that the planned referendum is no different to the situation in Scotland and Catalonia.