Ukraine's interim president, Oleksandr Turchynov, is calling on Russian President Vladimir Putin to stop what he calls "provocations" in Crimea.
Mr. Turchynov said Russian actions Friday were "naked aggression." He likened it to events that led up to Russia's 2008 invasion of Abkhazia -- a pro-Russian region of Georgia.
Ukraine's acting interior minister, Arsen Avakov, says Russian forces have taken control of the two main airports in Crimea. He calls this a "military invasion."
VOA correspondent Elizabeth Arrott, who flew into the airport of the Crimean capital of Simferopol Friday, saw gunmen in camouflage and carrying automatic weapons at the airport. But she described the situation as calm.
There are also reports of Russian troops surrounding the state-run television station in Simferopol. A Ukrainian official also says 2,000 Russian soldiers have landed at a military base near the capital.
Meanwhile, the Associated Press quoted Ukraine's State Border Guard Service as saying a Ukrainian coast guard base was surrounded by about 30 Russian marines.
The events come a day after gunmen seized control of government buildings in Crimea, a Ukrainian region with strong ties to Moscow. Those gunmen raised the Russian flag over the buildings.
The Crimean parliament voted Thursday to dismiss the regional government and hold a referendum to determine Crimea's status in Ukraine. The referendum is set for May 25, the same day Ukraine will hold a presidential election.
Crimea is a Black Sea peninsula in southern Ukraine which has a tiny border with Russia on its far eastern point. The late Soviet leader Nikita Khrushchev placed it under Ukrainian control in 1954 and it became part of Ukraine when the Soviet Union collapsed in 1991.
Much of the Crimean population is pro-Russian and Russia's Black Sea naval fleet has a headquarters in the Crimean city of Sevastopol. Crimea is also home to ethnic Muslim Tartars who generally show disdain towards Russia.
The Ukrainian parliament approved a new interim government Thursday led by new prime minister Arseniy Yatsenyuk.
Mr. Yatsenyuk has accused the government of ousted president Viktor Yanukovych of stealing $70 billion from the treasury and sending the money to offshore accounts. He also says $37 billion of credit it received has disappeared, leaving Ukraine with severe financial problems.
Judicial authorities in Geneva have opened an investigation into alleged money laundering by Mr. Yanukovych and his son. The Swiss government also announced it is freezing the assets of 20 Ukrainian officials.