Pro-Russian forces have stormed a Ukrainian air force base in Crimea, firing shots and smashing through gates and walls with armored vehicles.
The troops broke into the Belbek air base facing no apparent resistance from Ukrainian troops, some of whom sang the Ukrainian national anthem during the incident.
Ukrainian defense officials says at least one person was wounded.
Earlier Saturday, Russian forces issued an ultimatum for Ukrainian troops at the base to surrender. The Belbek base outside the Black Sea port city of Sevastopol has been one of the largest holdouts of Ukrainian military forces in Crimea.
Russian forces have been seizing Ukrainian military bases and warships in Crimea as Russia finalizes its annexation of the strategic peninsula. Ukrainian troops have offered minimal resistance thus far.
The Belbek air base shares facilities with Sevastopol's international airport. Russia also has a large naval base in Sevastopol.
Meanwhile, Russia said it hopes an international monitoring mission in Ukraine will be successful, but noted that foreign observers will be barred from Crimea, which Moscow says has become "part of Russia."
Russia and the Organization for Security and Cooperation in Europe agreed Friday to deploy civilian monitors to Ukraine.
The OSCE says up to 500 monitors will gather information on the security situation in Ukraine, including human rights. The United States says the OSCE has a mandate to work in all of Ukraine, including Crimea.
Separately, United Nations Secretary-General Ban Ki-moon praised Ukraine's new prime minister, Arseniy Yatsenyuk, for his "real leadership," on the second day of a visit to Ukraine. Mr. Ban also said he admired the prime minister's call for "inclusiveness and reconciliation" at a time when many Ukrainian citizens are angry and frustrated about developments in their country.
Russian President Vladimir Putin Friday signed a law completing the annexation of Crimea. The law recognizes parliament's approval of a referendum by Crimeans on breaking away from Ukraine.