28 February 2014
Last updated at 06:53 ET
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Christian Fraser says barriers and armed men are blocking Sevastopol airport
Ukraine's interior minister has accused Russian naval forces of occupying Sevastopol airport in the region of Crimea.
Arsen Avakov called their presence an "armed invasion".
But Russia's Black Sea Fleet has denied that Russian servicemen are taking part.
The other main Crimean airport, Simferopol, has also been occupied by armed men, thought to be pro-Russia militia.
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At the Scene
Sevastopol is by name an international airport, but civilian flights stopped some years ago, and it is owned by the Ukrainian Ministry of Defence.
So it would be of no real consequence that soldiers are guarding a military base were it not for the fact no-one knows whose orders they are obeying.
There are roadblocks springing up from here to the administrative capital Simferopol.
The local parliament is in session there, but is sharing the municipal building with a paramilitary unit, and Simferopol airport is also under protection.
There are also signs of further military movements:
- The BBC has seen eight truck with the black plates of the Russian army - usually the type used to carry personnel - moving towards Simferopol
- There are unconfirmed Ukrainian reports of eight Russian military helicopters arriving in Sevastopol
The Ukrainian parliament has called on the United Nations Security Council to discuss the unfolding crisis in Crimea.
As Ukraine's currency slides, the central bank has put a 15,000 hryvnia (1,000 euro; £820) limit on daily bank cash withdrawals.
Meanwhile interim President Olexander Turchynov has dismissed the Armed Forces chief, Yuriy Ilyin.
He was reported to have been admitted to hospital with a heart attack on Thursday.
Mr Ilyin was appointed earlier this month by Viktor Yanukovych, in one of his last acts before being ousted from the Ukraine presidency.
Tensions rise Relations between Russia and Ukraine have been strained since Mr Yanukovych lost power.
These tensions have been particularly evident in Crimea, Ukraine's only Russian-majority region.
The BBC's Bridget Kendall in Moscow says the Crimea is becoming the lynchpin of a struggle between Ukraine's new leaders and those loyal to Russia.
Mr Yanukovych is now in Russia and is preparing to hold a news conference on Friday in the city of Rostov-on-Don, near the Ukrainian border.
He surfaced in Russia on Thursday, asserting that he is still Ukraine's lawful president.
Ukraine's general prosecutor has said that Ukraine will ask Russia to extradite Mr Yanukovych, if it is confirmed that he is there.
Armed men, said by Mr Avakov to be Russian soldiers, arrived in the Sevastopol military airport near Russia's Black Sea Fleet Base on Friday morning.
The men were patrolling outside, backed up by armoured vehicles, but Ukrainian military and border guards remained inside, Mr Avakov said.
"I consider what has happened to be an armed invasion and occupation in violation of all international agreements and norms," Mr Avakov said on his Facebook page.
Armed men also arrived at Simferopol airport overnight, some carrying Russian flags.
A man called Vladimir told Reuters news agency he was a volunteer helping the group there, though he said he did not know where they came from.
"I'm with the People's Militia of Crimea. We're simple people, volunteers," he said.
"We're here at the airport to maintain order. We'll meet the planes with a nice smile - the airport is working as normal."
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The armed men at Simferopol Airport would not talk to the BBC
After the violent clashes and the ousting of Mr Yanukovych in Kiev, the focus of the Ukraine crisis has now moved to Crimea, which traditionally leans towards Russia.
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Crimea's airports
- Simferopol is the main international terminal, serving the regional capital
- Sevastopol, home to Russia's Black Sea Fleet, has a Soviet-era military airport (Belbek) which was also used for civilian flights until some years ago. Ukrainian air force jets are stationed there
- The Russian Black Sea Fleet has aircraft stationed at other air bases in Crimea (Gvardeyskaya and Kacha)
On Thursday, a group of unidentified armed men entered Crimea's parliament building by force, and hoisted a Russian flag on the roof.
The Crimean parliament later announced it would hold a referendum on expanding the region's autonomy on 25 May.
Russian President Vladimir Putin has urged his government to maintain relations with Kiev and even join Western efforts to bail out its troubled economy.
But he is also giving the Crimean government humanitarian aid.
The US sought assurances from Russia earlier this week, after Russian President Vladimir Putin ordered snap military drills to test the combat readiness of troops near the border with Ukraine.
US Secretary of State John Kerry has called on all sides to "step back and avoid any kind of provocations".
Mr Kerry said he had spoken to his Russian counterpart, Sergei Lavrov, who vowed to respect Ukraine's "territorial integrity".
Financial strain On top of its political problems, Ukraine also faces huge financial hurdles.
It says it needs $35 billion over the next two years to avoid default on its loans.
Russia has suspended the next instalment of a $15bn loan because of the political uncertainty.
Switzerland and Austria announced on Friday that it had launched an investigation against Mr Yanukovych and his son Aleksander for "aggravated money laundering".
Austria also said it had frozen the assets of 18 Ukrainians suspected of violating human rights and involvement in corruption. It did not give any names.
Crimea - where ethnic Russians are in a majority - was transferred from Russia to Ukraine in 1954.
Ethnic Ukrainians loyal to Kiev and Muslim Tatars - whose animosity towards Russia stretches back to Stalin's deportations during World War Two - have formed an alliance to oppose any move back towards Moscow.
Russia, along with the US, UK and France, pledged to uphold the territorial integrity of Ukraine in a memorandum signed in 1994.
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