Ukraine's interior minister has accused Russian forces of staging a "military invasion and occupation" after armed men took control of two airports in Ukraine's Crimea region.
Interior Minister Arsen Avakov wrote on his Facebook page Friday that armed Russian troops were blocking the Belbek airport in Sevastopol, where Russia has a naval base. Unidentified men were also patrolling the international airport in Simferopol, the Crimean capital.
Russian state media are carrying statements denying any Russian involvement.
Ukraine's Interim President Olekxandr Turchnyov has called an emergency session of his security chiefs over the situation.
The events come a day after gunmen seized control of government buildings in Crimea, a Ukrainian region with strong ties to Moscow. The gunmen also raised the Russian flag.
Meanwhile ousted Ukrainian president Viktor Yanukovych, 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.
Judicial authorities in Geneva on Friday launched a probe into the allegations of money laundering by Mr. Yanukovych and by his son. The Swiss government has also announced it is freezing the assets of 20 Ukrainian officials.
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.
Acting Ukrainian President Oleksandr Turchyno warned that any movement of Russian forces outside their base in the region 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.