Russian military forces have occupied an airport in the Black Sea port of Sevastopol in Crimea near the Russian naval base in an "armed invasion", Ukraine's interior minister has said.
On a day of escalating tensions between the two countries, eight Russian military helicopters were also reportedly seen flying towards the Belbek international airport in Sevastopol, although a video purporting to show the aircrafts landing has not been verified.
Another airport, Simferopol, has also been occupied by armed men, also thought to be pro-Russia militia. Eight army trucks with Russian plates were also moving towards Simferopol, the BBC has reported.
Ukraine's State Border Guard Service said about 30 Russian marines had taken positions outside its Coast Guard base in the Sevastopol base. The interior minister, Arsen Avakov said: "I consider what has happened to be a military invasion and occupation in violation of all international agreements and norms."
In response, Russia's Black Sea Fleet has denied its forces were involved in a seizure or blockade of the airport, according to the Interfax news agency.
"No units of the Black Sea fleet were deployed in the area of Belbek nor did they take place in blockading it," a statement from the fleet's press service said.
In other developments today, France, Germany and Poland released a joint statement saying they are "very worried" by the events and urged all parties to refrain from any action endangering Ukraine's territorial integrity.
The escalating conflict has sent Ukraine's finances plummeting, prompting Western leaders to prepare an emergency financial package. The International Monetary Fund has said it is “ ready to respond” to Ukraine's bid for financial assistance.
Ukraine's finance ministry has estimated it needs $35 billion (£21 million) over the next two years to avoid default.
Ukraine's new prime minister, Arseniy Yatsenyuk, said on Thursday that the country's future lies in the European Union, but with friendly relations with Russia.
Mr Yatsenyuk insisted the country wouldn't accept the secession of Crimea. The Black Sea territory, he declared, “has been and will be a part of Ukraine.”
The events follow warnings from the interim president that any movements of Russia's troops from their Crimea naval base will be considered an act of "military aggression" yesterday, after it scrambled fighter jets to patrol borders in the first stirrings of a potentially dangerous confrontation reminiscent of Cold War brinksmanship.
The exercise came a day after US Secretary of State John Kerry warned Russian President Vladimir Putin to remember that the escalating crisis "is not Rocky IV".
Ukraine's population is divided in loyalties between Russia and the West. Crimea became part of Ukraine in 1954 when Soviet leader Nikita Khrushchev transferred jurisdiction from Russia.