RUSSIAN President Vladimir Putin demanded and won his parliament's approval at the weekend to invade Ukraine.
Putin's open assertion of the right to send troops to a country of 46 million people creates the biggest confrontation between Russia and the West since the Cold War.
Troops with no insignia on their uniforms but clearly Russian, some in vehicles with Russian number plates, have already seized Crimea, an isolated peninsula in the Black Sea where Moscow has a large military presence in the headquarters of its Black Sea Fleet.
Kiev's new authorities have been powerless to stop them.
US President Barack Obama told Russian President Vladimir Putin that his country is violating international law by sending troops into Ukraine.
The White House said Obama "expressed his deep concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity."
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