Armed pro-Russian activists have stormed the headquarters of Ukraine’s navy in the Crimean city of Sevastopol.
Several Ukrainian servicemen have left, the Russia flag is flying and there are reports that Ukrainian navy chief Serhiy Hayduk has been detained.
It comes a day after Crimean leaders signed a treaty with Russia absorbing the peninsula into Russia.
Sunday’s disputed referendum, which officials say backed splitting from Ukraine, has been widely condemned.
Crimean and Russian officials say the vote showed overwhelming public support for joining Russia, with 97% of voters in favor.
But the West and the Ukrainian government in Kiev say the referendum – organized in two weeks and boycotted by many of Crimea’s Ukrainian and Tatar minorities – was illegal, and the results will not be recognized.
Activists put Russian flag at the Sevastapol navy base
On Wednesday, Russia’s constitutional court approved the treaty as legal.
Ukrainian Defense Minister Ihor Tenyukh has reportedly been ordered to head to Crimea amid the rising tensions.
Russia’s Interfax news agency quoted Crimea’s Prime Minister Sergei Aksyonov, who is in Moscow, as saying: “Nobody will let them into Crimea, they will be sent back.”
Later, there were reports that Ukrainian navy chief Serhiy Hayduk had been detained and taken away from the base by the Russian Federal Security Service (FSB).
A number of Ukrainian servicemen were seen leaving the base. There have been no reports of clashes or shots being fired.
Ukrainian navy spokesman Sergiy Bogdanov told AFP news agency: “There are about 200 of them, some wearing balaclavas.”
“They are unarmed and no shots have been fired from our side. The officers have barricaded themselves inside the building,” he said.
He said even though Kiev had authorized the military to use force in Crimea in self-defense “we are not doing so and will not do so”.
Reports are also emerging of a similar incident at a Ukrainian navy base in Novo-Ozyorne, western Crimea.
Ukrainian defense ministry spokesman Vladyslav Seleznev said a tractor had rammed the gates of the compound and pro-Russians had entered the base.
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 took over Simferopol airport in Crimea on Friday morning
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.