Pro-Russian activists, some of them armed, have stormed the headquarters of the Ukrainian navy in the Crimean city of Sevastopol.
Russian flags have been seen flying from buildings and several Ukrainian servicemen were seen leaving the base.
It comes a day after Crimean leaders signed a treaty with Russia absorbing the peninsula into the Russian Federation after a disputed referendum.
The move has been widely condemned internationally.Crimean and Russian officials say the referendum showed overwhelming public support for joining Russia.
But the Ukrainian government in Kiev and the West say the vote – organised in two weeks and boycotted by many of Crimea’s Ukrainian and Tatar minorities – was illegal, and the results will not be recognised.
On Monday, the US and the EU imposed sanctions on several officials from Russia and Ukraine accused of involvement in Moscow’s actions in the Black Sea peninsula.
Brussels and the White House have said the sanctions will be expanded now that the treaty on Crimea has been signed.
Moscow has warned this is “unacceptable and will not remain without consequences”.
Officers ‘negotiating’
An officer inside the Sevastopol base told the BBC that some 200 pro-Russian activists had broken down the gates and gone inside, using an ambulance car.
The senior Ukrainian officer at the scene was negotiating with them, he said.
A number of Ukrainian servicemen were later seen leaving the base. There have been no reports of clashes or shots being fired.
The port city of Sevastopol is home to the Russian Black Sea fleet.
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 authorised the military to use force in Crimea in self-defence “we are not doing so and will not do so”.
The Ukrainian crisis began last November after pro-Russian President Viktor Yanukovych ditched an EU association agreement in favour of stronger ties with Moscow.
He fled Ukraine on 22 February after protests in which more than 80 people were killed.
Crimea has been part of Ukraine since 1954 but has a predominantly ethnic Russian population.
Pro-Russian forces effectively took control of the peninsula in late February and gunmen seized buildings, including the parliament.
The prime minister was removed from office on 27 February by a vote of no confidence and replaced by pro-Moscow Sergei Aksyonov, leader of the small Russian Unity party. – BBC News
