According to Ukraine’s Interior Minister Arsen Avakov, Russian military forces are blockading Sevastopol airport in the Ukrainian region of Crimea.
Arsen Avakov called their presence an “armed invasion”.
Armed men also took over the other main Crimean airport, Simferopol, on Friday morning.
Relations between Russia and the Ukraine have been strained since the ousting of Ukrainian President Viktor Yanokovych, who is now in Russia.
These tensions have been particularly evident in Crimea, Ukraine’s only Russian-majority region.
On Thursday, pro-Russian armed men stormed the Simferopol parliament, ousted the existing cabinet and appointed a new prime minister.
Meanwhile, in a further challenge to Kiev, Viktor Yanukovich is preparing to give a press conference on Friday, after resurfacing in Russia on Thursday, asserting that he is still Ukraine’s lawful president.
Armed men, said by Arsen 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 armored vehicles, but Ukrainian military and border guards remained inside, Arsen Avakov said.
“I consider what has happened to be an armed invasion and occupation in violation of all international agreements and norms,” Arsen Avakov said on his Facebook page.
Armed men also arrived at Simferopol airport overnight, some carrying Russian flags.
A man called Vladimir told Reuters 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.”
On Thursday, a separate 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 May 25.
Recent developments in the Crimea region – which traditionally leans towards Moscow – heightened tensions with Russia, which scrambled fighter jets to monitor its borders on Thursday.
Russia’s President, Vladimir Putin, last night urged his government to maintain relations with Kiev and even join Western efforts to bail out its troubled economy – but he is also rewarding the rebellious Crimean government with humanitarian aid from Russia.
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