Ukraine's interior minister Arsen Avakov says Russian forces were patrolling the airport in the capital of the Crimea region in what he called an armed invasion and occupation.
The men are reported to have arrived at the airport in Simferopol on Friday.
Earlier reports said flights and passengers appeared to be moving normally in and out of the airport.
Meanwhile ousted Ukrainian president Viktor Yanukovich, who says he is under Russian protection, is expected to address the media on Friday in a news conference in the southern Russian city of Rostov-on-Don.
Late Thursday, U.S. Vice President Joe Biden telephoned Ukraine's Prime Minister Arseniy Yatsenyuk to welcome the formation of a new government and pledge U.S. support as Ukraine undertakes reforms aimed at restoring the country's economic health. Ukraine's parliament approved Mr. Yatsenyuk, a popular, pro-Western opposition leader, as the head of the interim government earlier in the day.
Mr. Yatsenyuk has accused the Yanukovych government of stealing billions from the state treasury. He said $70 billion in Ukrainian government money had been sent to offshore accounts over the last three years, and that $37 billion of credit it received has disappeared, leaving Ukraine with severe financial problems.
The International Monetary Fund and European Union are sending teams to Ukraine to assess the country's needs. The United States also is considering $1 billion in loan guarantees.
Also Thursday, the Kremlin sent Russian fighter jets to patrol its border with Ukraine -- a day after announcing large-scale military exercises in the area.
Gunmen seized control of government buildings in Crimea -- a Ukrainian region with strong ties to Moscow and home to a Russian naval base in Sevastopol. The gunmen raised the Russian flag.
Acting Ukrainian President Oleksandr Turchyno warned that any movement of Russian forces outside the base would be considered "military aggression."
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.