President Obama has ruled out US military intervention in Ukraine.

That's despite promising to "ratchet up" the diplomatic pressure on Russia, following the annexation of Crimea.

UN Secretary General Ban Ki-moon's in the region today, to try to find a peaceful solution to the crisis.

Meanwhile, European Union leaders are likely to extend asset freezes and travel bans on key members of the Russian regime, as they meet in Brussels to discuss the Ukraine crisis.

NATO Secretary-General Anders Fogh Rasmussen has warned Russia's advances in Ukraine are the greatest threat to European security since the Cold War: