Ukraine put its armed forces on the highest alert on Sunday after Russian President Vladimir Putin declared he had the right to use military force to protect Russian citizens in the country, creating the biggest confrontation between Moscow and the West since the Cold War.

"This is not a threat, this is actually the declaration of war to my country," said Ukraine's Prime Minister Arseny Yatsenyuk, head of a pro-Western government that took power when Russian ally Viktor Yanukovych fled last week.

Putin obtained permission from Russia's parliament on Saturday to send troops into Ukraine, spurning Western pleas not to intervene.