As many as 50 armed men seized the regional government headquarters in Crimea on Thursday and raised the Russian flag.
The region provides a base for the Russia's Black Sea Fleet.
Ukraine's new government in Kiev has warned that any movement by Russian forces beyond the base's territory would be tantamount to aggression.
Russian Foreign Minister Sergei Lavrov moved to assure U.S. Secretary of State John Kerry that Russia will respect Ukraine's territory.
In response, the U.S. told Russia to show it's sincere about its promise.
Washington is concerned after Russian President Vladimir Putin ordered a surprise military exercise on Ukraine's doorstep this week.
Other Western nations, like Britain and Germany, are also calling on Russia to help ease the turmoil in Ukraine's Russian-dominated region.
Also Thursday, the new Ukrainian government held its first Cabinet meeting led by newly-appointed Prime Minister Arseny Yatseniuk.
In the meantime, ousted president Viktor Yanukovich is said to be in Russia, as Russian state news agencies reported he plans to hold a news conference on Friday.
He has declared he is still Ukraine's president, but has lost support across almost the entire country.
Shin Se-min, Arirang News.
Now in further developments, Russian soldiers have reportedly taken over two airports in the pro-Russia region of Crimea.
Interior Minister Arsen Avakov denounced the move as an armed invasion that's in violation of international agreements.
It remains unclear whether the seizure is the initial stage of a larger military operation or if it's to simply stall Ukranian authorities from exerting greater control over the autonomous region.