Troops stormed a Ukrainian base on the outskirts of Simferopol on Tuesday, according to a Ukrainian military spokesman.
One Ukrainian officer was initially reported to have been wounded in the neck. The Interfax and Reuters news agencies reported that a soldier had been killed.
Kyiv, which blamed Russia for the attack, authorized Ukrainian troops to use weapons if necessary to defend themselves.
A reporter for the AFP news agency said a burst of gunfire had been heard from a Ukrainian military unit in a suburb northeast of Simferopol. Two ambulances were seen driving into the area.
Immediately after the incident, Ukrainian Prime Minister Arseniy Yatseniuk said the conflict had moved from a political to a military stage.
"Today, Russian soldiers began shooting at Ukrainian servicemen," said Yatseniuk. "This is a war crime."
Earlier in the day, Russian President Vladimir Putin signed a treaty with Crimean leaders that made the region part of the Russian Federation.
A crowd in Red Square chanted "Russia" and "Putin" after the treaty was signed. Putin addressed them later telling them Russia had returned to "home port."
"Crimea and Sevastapol are returning home to their home shores, to their home port, to Russia," he said.
Ukraine's interim President Oleksandr Turchynov has said Kyiv will never recognize the move.
rc/ccp (AP, AFP, Reuters)