A Ukrainian soldier digs a trench on a checkpoint not far of the border with Crimea near the village of Strilkove

Ukraine's government says Russian forces stormed a military base in Crimea and killed a Ukrainian officer, escalating tensions in the region

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—whom he said stormed a Ukranian 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 AFP. “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.