There's been swift reaction to a vote by the Crimean government, to become part of Russia.
The question will be put to a public referendum in the region on the 16th of this month.
But the Ukrainian government has sovereignty over Crimea, so it would have to agree.
Interim Prime Minister Arseniy Yatsenyuk says the move is illegitimate, and he's urged Russia "not to support those who advocate separatism".
He says Russia has no legal grounds or reason to go in.
"One country which posseses nuclear weapons, just decides in a snapshot to invade another."
And NATO is standing by Ukraine, after the local government for Crimea voted to ask Russia to come in and take over.
The Ukrainian government must agree to that move for it to happen - and it clearly doesn't.
NATO Secretary General Anders Fogh Rasmussen says there should be no attempt to draw new lines on the map of Europe, in the 21st Century.
"We call on Russia to honour its international commitments and halt the military escalation in Crimea.
"We call on Russia to withdraw its forces to their bases."
The United States has stepped up its response to Russia's occupation of the Crimea.
President Obama has blocked visas for Russian opponents of the new government in the Ukraine.
US correspondent Richard Arnold says Mr Obama has also paved the way for financial sanctions.
"The white house not spelling out yet who is included in these new sanctions except that they will not apply to President Putin, it would be unusual to sanction a head of state."
Richard Arnold says US Secretary of State John Kerry is continuing to hold talks in Paris with his Russian counter part Sergey Lavrov.