Pro-Russian activists have entered a Ukrainian naval base in the Crimean city of Sevastopol.
Russian flags have been seen flying from buildings, the headquarters of the Ukrainian navy.
It comes a day after Crimean leaders signed a treaty with Russia absorbing the peninsula into the Russian Federation after a disputed referendum.
The government in Kiev, the EU and the US, among others, have said they will not recognise the move.
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.
An officer inside the Sevastopol base told the BBC that some 200 pro-Russian activists had broken down the gates of the base and gone inside, using an ambulance car.
The senior Ukrainian officer at the scene was negotiating with them, he said.
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 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.
Pro-Russia armed men then took effective control of Crimea, which has been part of Ukraine since 1954 but has a predominantly ethnic Russian population.
The Crimean port city of Sevastopol is home to the Russian Black Sea fleet.