Kiev, Ukraine: Ukraine is ready to fight for its sovereignty, Prime Minister Arseniy Yatsenyuk declared in Kiev, after pro-Russian forces seized control of the Black Sea region of Crimea, triggering one of the biggest crises in Russia's relations with the West since the end of the Cold War.

Ukraine told Russia that a military invasion would be an act of war following a vote by lawmakers in Moscow to give President Vladimir Putin the right to send troops into the neighbouring country. The Ukrainian government, which put its military on full combat alert, is taking "all measures" to ensure peace and unity, Mr Yatsenyuk said.

On Sunday, Ukraine announced it would call up all military reservists as witnesses said a group of Russian soldiers had blocked about 400 Ukrainian marines at their base in the eastern Crimean port city of Feodosiya and were calling on them to surrender.

The head of the National Security and Defence Council Andriy Parubiy told reporters that the council had ordered the Defence Ministry to "call on all those that armed forces need at the moment across Ukraine," adding that the mobilisation was to "ensure the security and territorial integrity of Ukraine".