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.
Meanwhile, Ukraine's acting president Olexander Turchynov signed a decree dismissing the head of the armed forces general staff, his office said on Friday. Admiral Yuriy Ilyin was only appointed at height of protests against the ousted president Viktor Yanokovych.
The Ukrainian parliament has asked the UN Security Council to discuss the escalating situation in Crimea and Mr Turchynov has called an emergency session of security chiefs to discuss the situation in Crimea.
He proposed parliament hold a vote to adopt an appeal to Russia to "respect Ukraine's territorial integrity" and "reject support for separatism in Ukraine, of any form".
In pictures: Crisis in Ukraine
In pictures: Crisis in Ukraine
1/11 Demonstrations in Crimea
Crimean Tatars drag away a police officer in front of a local government building in Simferopol
2/11 Demonstrations in Crimea
Pro-Russian activists pray outside the Crimean Parliament building in Simferopol
3/11 Demonstrations in Crimea
Crimean Tatars hold flags during rallies near the Crimean parliament building in Simferopol
4/11 Demonstrations in Kiev
Young girls chant the song 'Glory to Ukraine' at Independence Square in central Kiev
5/11 Demonstrations in Crimea
A man receives medical treatment after he was injured in clashes during rallies near the Crimean parliament building
6/11 Demonstrations in Crimea
Pro-Ukrainian activists hold placards reading "Crimea +Ukraine is love" during a rally in front of the Crimean parliament in Semfiropol
7/11 Demonstrations in Kiev
Protestors capture a military armoured vehicle in central Kiev
8/11 Demonstrations in Kiev
An old man stands with an Ukrainian flag on Kiev's Independence Square
9/11 Demonstrations in Kiev
A man lays flowers at one of the barricades heading to Kiev's Independence Square
10/11 Demonstrations in Kiev
Flowers cover the ground and barricades where protesters were killed in a recent clash with riot police in Kiev's Independence Square
11/11 Demonstrations in Kiev
A self-defense unit patrolling the city centre in Kiev, Ukraine
The general prosecutor's office has said it will ask Russia to extradite Mr Yanukovych if it is confirmed that he is in the country. Authorities in Switzerland announced they have launched a corruption probe against him and blocked all potential assets he may have tried to hide.
In addition, Ukraine's central bank placed a 15,000 hryvnia (1,000 euro; £820) limit on daily bank withdrawal, the BBC has reported.
Separately, dozens of armed men in military uniforms without markings occupied Simferopol, the other main airport in Crimea in the early hours of this morning. A spokesman for the airport said it is operating as normal.
Relations between Russia and Ukraine have become increasingly strained following the ousting of Mr Yanokovych, particularly in Crimea, Ukraine's only region with an ethnic Russian majority.
Russian state television quoted eyewitnesses saying the men arrived at the Simferopol airport in the early hours.
It was not immediately clear who the men were, as they were wearing uniforms without any insignia.
One man told the BBC: "I'm with the People's Militia of Crimea. We're simple people, volunteers.
"We're here at the airport to maintain order. We'll meet the planes with a nice smile - the airport is working as normal."
On Thursday, masked gunmen with rocket-propelled grenades and sniper rifles seized the parliament and government offices in Simferopol and raised the Russian flag over the parliament building.
Ukraine's parliament elected a new government later that day, led by Mr Turchynov, who promptly pledged to prevent any national division.
Russia also has granted shelter to Ukraine's fugitive president, Viktor Yanukovych, it emerged yesterday, who is wanted by the new government over the violence that led to the deaths of over 80 people during three days of bloody clashes.
Mr Yanukovych declared on Thursday in a statement that he remains Ukraine's legitimate president.
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.
Additional reporting by Reuters and The Associated Press