Ukraine's military says an officer has been killed in an attack on a base in Crimea, the first such death since pro-Russia forces took control in February.
It said all its troops at the Simferopol base had been arrested.
The attack came shortly after Russian President Vladimir Putin and the leaders of Crimea signed a bill to absorb the peninsula into Russia.
Western powers condemned the treaty and a G7 and EU crisis meeting has been called for next week in The Hague.
The Ukrainian crisis began in November last year after pro-Moscow President Viktor Yanukovych abandoned an EU deal in favour of stronger ties with Russia. He fled Ukraine on 22 February after deadly protests.
The Ukrainian defence ministry said a warrant officer who was on duty in a park inside the Simferopol base had been killed and another serviceman injured.Spokesman Vladislav Seleznyov told Reuters the attack was by "unknown forces, fully equipped and their faces covered".
The Ukrainians had their IDs, weapons and money confiscated, he said.
Interim Prime Minister Arseniy Yatsenyuk told an emergency government meeting: "The conflict is shifting from a political to a military stage.
"Russian soldiers have started shooting at Ukrainian military servicemen and that is a war crime."
Reports from the Crimean news agency, Kryminform, said a pro-Russian defence force member had also been shot dead in a separate incident in Simferopol.
Neither of the deaths in the two incidents has been independently confirmed.