Ukrainian soldiers serving in Crimea have been authorized to use their weapons to defend themselves, according to the acting Ukrainian president's press service.

The order comes only hours after a Ukrainian serviceman was shot and killed on Tuesday at a Ukrainian base that came under attack in Crimea's main town of Simferopol and acting Ukrainian Prime Minister Arseny Yatseniuk said the Crimean conflict has now entered a "military phase."

Until now, forces deployed on the Black Sea peninsula, taken over three weeks ago by Russian forces, had been told to avoid using arms against attack.

A defence ministry order issued after the incident said: "In connection with the death of a Ukrainian serviceman ... Ukrainian troops in Crimea have been allowed to use weapons to defend and protect the lives of Ukrainian servicemen."

Unknown assailants

Vladislav Seleznyov, speaking to Reuters by telephone from Crimea about the earlier assault, said one serviceman at the base had died of his wounds. A second man, a captain, was injured.

Seleznyov said it was unclear who had staged the assault, but described the attackers, as "unknown forces, fully equipped and their faces covered".

Yatseniuk also accused Russia of committing a "war crime" by firing on Ukrainian servicemen.

"The conflict is moving from a political one to a military one because of Russian soldiers," he told a meeting at Ukraine's defence ministry.

"Today, Russian soldiers began shooting at Ukrainian servicemen and this is a war crime without any expiry under a statute of limitations."

Yatseniuk said he had ordered Ukraine's defence 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."

Ukraine, fearing a large-scale Russian invasion, has called upon 20,000 Ukrainians with some degree of military training to report for service. On Tuesday, groups of reservists trained outside Kyiv.

Ukrainian officials said there are more than 20,000 Russian soldiers on its borders — troops Russia claims are merely training.