Ukrainian Prime Minister Arseny Yatseniuk said on Tuesday that the conflict in its Crimea peninsula, now under Russian control, had entered a military phase after an Ukrainian officer was killed in an alleged Russian shooting at a Crimea base.

"The conflict is moving from a political one to a military one because of Russian soldiers," he told a meeting at Ukraine's defense ministry. "Today, Russian soldiers began shooting at Ukrainian servicemen and this is a war crime without any expiry under a statute of limitations."

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Yatseniuk said he had ordered Ukraine's defense minister to call a meeting with his counterparts from Britain, France and Russia - signatories to a 1994 treaty guaranteeing Ukraine's borders to "prevent an escalation of the conflict."

Earlier, a military spokesman said a Ukrainian officer was killed in a shooting at a military facility on the outskirts of the Crimean capital Simferopol, but it was unclear who was behind the incident.

Following the incident, Ukraine issued orders permitting its soldiers in Crimea to use weapons to protect their lives, Acting President Oleksander Turchinov's press service said. Until now, forces deployed on the Black Sea peninsula had been told to avoid using arms against attack.

Some Ukrainian military facilities in Crimea have been under the control of Russian forces for several weeks after Russian troops poured into the Black Sea peninsula ahead of a referendum at the weekend that handed over control from Ukraine to Russia.

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There was no immediate evidence that Russian soldiers were involved in Tuesday's incident, witnesses said, and there are contradictory accounts about the identity of the victim, with Ukrainian media claiming he is a Ukrainian soldier, and pro-Russian sources saying he is a member of the local "self defence forces."

It was not possible to see far into the compound, because streets leading to it had been blocked by so-called "self-defense" units of pro-Russian volunteers who have been patrolling the streets of Crimea in the run-up to the referendum.

There have been major concerns over whether the governments of Russia and Ukraine, who are at loggerheads over Moscow's annexation of Crimea, could achieve a smooth handover of control of Ukrainian military bases in the region.

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Pro-Russian nationalists have been seen gathering outside Ukrainian military bases on several occasions this month, demanding that Ukrainian soldiers leave and hand over control to Russian forces.

The fact that the road blocks outside the facility in Simferopol were staffed by "self-defense" units suggested it was they who were involved in the incident rather than Russian forces. The situation was calm by 1520 GMT.

On Sunday, the Ukrainian and Russian Defense Ministries said they had agreed on a truce.

