Tensions in the disputed region of Crimea reached new heights Tuesday as Ukraine said a military officer was killed shortly after Russia formally annexed the breakaway peninsula.

Interim Ukrainian Prime Minister Arseniy Yatsenyuk blamed Russian forces — who he said stormed a Ukrainian military outpost near the city of Simferopol — for the killing, and warned the conflict is “shifting from a political to a military stage.”

“Today, Russian soldiers began shooting at Ukrainian servicemen,” he said, according to Agence France-Presse. “This is a war crime.”

His claim could not be immediately verified, but if true, it would mark the first fatality in clashes between the two countries’ militaries during the weeks-long crisis in Crimea.

Russian President Vladimir Putin, delivering a televised address to the country’s parliament on Tuesday, defended his country’s decision to absorb the region after it voted Sunday to split from Ukraine.

“Crimea has always been an integral part of Russia in the hearts and minds of people,” Putin said.

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