Russia hasn't sought UN support as they have the support of the President of Ukraine currently in Moscow, who they argue is the legitimate President and the governor (or equivelent) of Crimea, hence they can claim it is not a breach of territorial integrity or sovereignty as per Art 2(7) of the UN Charter.
The UN would be deadlocked on the matter, with the US and Russia in opposing camps. More and more since the unilateral invasion of Iraq by the US without UN support and threats of action against Iran and Syria by the West, it is beginning to see to be legitimate to act unilaterally. The US opened a big can of worms by committing to action outside of UN support, now that other nations see similar actions as legitimate it appears like hypocrisy for the West or the US in particular to be critical when other nations wish to do the same.
Essentially Russia are looking after her own interests in the region. I guess it makes a change to US interventionalism.