Ukraine said one of its soldiers was killed in Crimea Tuesday in the first case of bloodshed since Russian troops and pro-Kremlin militia seized the rebel peninsula almost three weeks ago.
"The conflict is shifting from a political to a military stage," Ukraine's Prime Minister Arseniy Yatsenyuk told an emergency government meeting.
"Russian soldiers have started shooting at Ukrainian military servicemen, and that is a war crime," he said at the nationally televised session.
He was speaking after President Vladimir Putin signed a treaty claiming Crimea as Russian territory following the Black Sea region's overwhelming vote on Sunday in favour of switching from Ukrainian to Kremlin rule.
Regional defence ministry spokesman Vladislav Seleznyov told AFP the soldier had died after being shot in the neck when a group of gunmen stormed a Ukrainian military base in the northeast of Crimea's main city of Simferopol.
Seleznyov said another soldier was wounded but did not specify whether the base was stormed by Russian soldiers or pro-Kremlin militia who also patrol the peninsula.
The defence ministry spokesman said pro-Russian forces had by late Tuesday taken complete control of Ukraine's Simferopol base.
"The centre has been taken under their full control. All the servicemen inside were lined up in a row and their documents seized," he said.
"They were all informed that they were under arrest."
Seleznyov said he could not immediately say how many Ukrainian soldiers had been arrested or the number of pro-Russians involved in the attack.
There was no immediate reaction to the reported death from either Russian authorities in Moscow or the peninsula's rebel leadership.
Russian forces took de facto control of the peninsula at the beginning of March after the toppling last month of the pro-Kremlin regime in Ukraine and the rise to power of a new Western-backed administration that is seeking closer ties with the European Union.
Ukraine's navy chief Sergiy Gayduk had told the same government meeting that an officer had been shot and injured in the leg "during an attack against a base in Simferopol."
He did not specify where or when the attack happened or who was behind it and it was not immediately clear if it was the same incident.
An AFP reporter outside a Ukrainian military unit in a suburb northeast of Simferopol heard a burst of gunfire coming from the building and saw two ambulances driving into the area
The region around the military unit was sealed off by what appeared to be pro-Moscow militants.
"Armed attempts to take over (Ukrainian) military units have multiplied in recent days," Gayduk said.