About 200 pro-Russian protesters on Saturday stormed a Ukrainian air force base in western Crimea, according to correspondents for the French news agency AFP.
The unarmed crowd broke through to the base in the town of Novofedorivka and started smashing windows as Ukrainian servicemen barricaded themselves inside buildings and threw smoke bombs at the intruders from the roof of the air base building, which is north of Sevastopol.
Ukrainian troops have barricaded themselves inside building, throwing smoke bombs.
Meanwhile an international group of observers is due to arrive in Ukraine on Saturday to monitor the security and human rights situation for at least six months.
The Organisation for Security and Cooperation in Europe will eventually deploy a total of 100 observers throughout Ukraine in hopes of "reducing tensions and fostering peace, stability and security." They will also check that the rights of ethnic minorities are being protected.
But they will not enter the contested region of Crimea, a statement from the Russian Foreign Ministry said, which said the disputed region has now "become part of Russia".
Moscow annexed the territory this week following a controversial snap referendum there that produced an overwhelming majority of votes in favour of leaving Ukraine to join Russia.
Ukraine, the United States and the EU insist that the Black Sea peninsula is still a part of Ukraine.