One Ukrainian soldier was killed and an officer wounded in an incident at a base near Simferopol, the Crimean regional capital.

In a statement, Ukraine’s Ministry of Defense said the assailants “were wearing military uniforms of the Armed Forces of the Russian Federation.” The ministry said it had now authorized its soldiers in Crimea to respond with live fire in self-defense.

The incident Tuesday afternoon began when unidentified assailants “stormed” the base, said Vladislav Seleznyov, a spokesman for the Ukrainian military. It’s unclear whether they were part of the Russian military or a volunteer militia.

He said guns were fired into the air, and it is unclear whether the two victims were struck by stray bullets or if they were fired at directly.

Seleznyov said a captain was wounded and taken to a hospital for treatment. The other Ukrainian soldier died of his wounds. His rank was not available.

Seleznyov said a truck flying a Russian flag was blockading the base’s gate. Many vehicles in Crimea have Russian flags nowadays, and the presence of one is not necessarily an indication that the truck belongs to the Russian military.

Citing the storming of the base in Crimea, Ukrainian Prime Minister Arseniy Yatsenyuk said that the conflict with Russia had moved from a “political to a military phase.”

A spokeswoman for Yatsenyuk’s office confirmed that one soldier was killed another was wounded. She said Yatsenyuk had called for top defense officials from Ukraine, the United States, Britain and Russia — signatories of the Budapest Memorandum, or the 1994 document guaranteeing Ukraine’s territorial integrity for surrendering its Soviet-era nuclear weapons — to urgently convene to defuse the crisis.

BBC reporter Ben Brown, from a base in Simferopol, reported gunfire:

“Despite the armed aggression of Russia against Ukraine, I will do everything possible not only to keep the peace but also to build a genuine partnership with Russia and good neighbor relations,” Yatsenyuk said.

Morello reported from Crimea and Faiola from Kiev, Ukraine. Terri Rupar in Washington contributed to this report.