Russian troops have blockaded Crimea's Belbek air base outside the Black Sea port city of Sevastopol, and they are said to be calling on Ukrainian forces there to surrender.
Russian military units moved in to surround Belbek Saturday without any advance announcement. They would not discuss their mission there and, in at least one case, seized a news crew's cameras and told reporters to leave.
Reuters news agency reports it contacted a Ukrainian commander at Belbek as saying the Russian forces issued an ultimatum for Ukrainian troops to surrender.
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.
VOA correspondent Daniel Schearf said Russian troops seized his crew's camera equipment outside the Belbek base Saturday, after the crew videotaped Russian armored personnel carriers gathering there.
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, U.N. Secretary-General Ban Ki-moon praised Ukraine's new Prime Minister Arseniy Yatsenyuk for his "real leadership," on the second day of his 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 signed a law Friday completing the annexation of Crimea. The law recognizes parliament's approval of a referendum by Crimeans on breaking away from Ukraine.