Ukrainian troops and police officers are on high alert, after armed men seized government buildings in the Crimea region of the country and raised a Russian flag over a barricade reports the Al Jazeera.
Acting president Oleksander Turchinov told parliament on Thursday he had given an order for security structures to take "all measures" needed to protect citizens.
Part of Russia's Black Sea fleet is based in Crimea, in the port of Sevastopol. Turchinov told Russian forces in Crimea not to leave their base, saying any movement would be considered a "military aggression".
The escalation in tension and rhetoric between Russia and Ukraine has steadily increased during the past months, but the overnight armed takeover could be a turning point in relations between the two countries.
Interior minister Arsen Avakov said in a Facebook post on Thursday that "unknown men armed with automatic and machine guns seized the building of the Crimean government in Simferopol. Interior troops and the entire police force have been put on alert". The area has been cordoned off "to prevent bloodshed", according to news agency AFP.
"Measures have been taken to counter extremist actions and not allow the situation to escalate into an armed confrontation in the centre of the city,'' Avakov said.
Pictures from Twitter, also posted on Thursday, showed the Russian flag raised above the Crimean parliament building. Up to 50 men with weapons marched into the buildings during the night.
There has been no formal response from Moscow to the latest developments in Crimea, but Russia's defence ministry has said its fighter jets are on standby. The country's foreign ministry said on Twitter it would "continue to defend in the international arena the rights of its compatriots, it will strongly and uncompromisingly react when they are violated". It also said that "large-scale human rights violations" in Ukraine were a cause for concern.
Crimean Prime Minister Anatoliy Mohilyov told AFP that local authorities were preparing to "take measures". He did not say what they were.
The men, in full combat dress but without any markings of affiliation entered the buildings and removed the guards without any fight, the Interfax-Ukraine news agency quoted sources in parliament as saying.
Scuffles in Crimea
On Wednesday there were scuffles outside the Crimea regional parliament building between thousands of pro-Russia separatists and supporters of Ukraine's new leaders, as regional politicians prepared to debate the removal of Viktor Yanukovich from the presidency.
Interfax news agency quoted a witness as saying there were about 60 people inside and they had many weapons. It said no one had been hurt when the buildings were seized by Russian speakers.
One, an ethnic Russian called Leonid Khazanov, told news agency Reuters: "We were building barricades in the night to protect parliament. Then this young Russian guy came up with a pistol...we all lay down, some more ran up, there was some shooting and around 50 went in through the window.
"They're still there ... Then the police came, they seemed scared. I asked them (the armed men) what they wanted and they
said 'To make our own decisions, not to have Kiev telling us what to do'," said Khazanov.