Ukraine's acting president warned Moscow to keep military personnel in his country confined to their naval base after suspected pro-Russian gunmen seized a regional government headquarters and parliament in the Crimea region.
"I am appealing to the military leadership of the Russian Black Sea fleet," said Olexander Turchinov, Ukraine's acting president since the removal of Viktor Yanukovich last week. "Any military movements, the more so if they are with weapons, beyond the boundaries of this territory (the base) will be seen by us as military aggression."
Russian armored personnel carriers and trucks carrying troops have been seen in the streets of the Crimean city of Sevastopol this week.
An agreement allows the Russian Black Sea fleet basing rights at the Ukrainian port of Sevastopol, which is located 500 miles south of Kiev, until 2042. Crimea is the only Ukrainian region with a Russian-speaking majority.
It was not immediately known who was occupying the buildings Thursday in the regional capital Simferopol but witnesses said they spoke Russian and appeared to be Russian separatists. They also raised a Russian flag.
Russia has repeatedly expressed concern for the safety of Russian citizens in Ukraine, using language similar to statements that preceded its invasion of Georgia in 2008.
On Wednesday, Russian President Vladimir Putin ordered 150,000 troops to be ready for war games near Ukraine.
And in remarks reported by state news agency RIA, Russian Defense Minister Sergei Shoigu said Russia was "carefully watching what is happening in Crimea" and taking "measures to guarantee the safety of facilities, infrastructure and arsenals of the Black Sea fleet."
Ukraine is ethnically homogenous, and does not allow dual citizenship. However, there is a clear east-west split in its political and economic outlook.
Wealthy western Ukraine looks toward its European neighbors – especially Poland - and this is where Ukrainian nationalist sentiment is strongest.
In the industrialized east and south, ties are closer to Russia. In some states, most notably Donetsk, Luhansk and Crimea, Russian is the primary language and the outlook is pro-Russian.
Putin ordered the invasion of neighboring Georgia in 2008 to protect two self-declared independent regions with many ethnic Russians and others holding Russian passports, and then recognized the regions as independent states.
Jason Cumming and Alastair Jamieson of NBC News, The Associated Press and Reuters contributed to this report.