Ukraine's new leadership has accused Russia of staging an "armed occupation and invasion of a sovereign state", as Russian politicians were set to debate a new law which makes it easier for new states to join the Russian Federation.
Gun-toting militants, dressed in military fatigues have now taken control of two airports in the Crimea, a Russian-speaking area of Ukraine, as well as occupying the parliament building, where they raised the Russian flag.
Dozens of armed men in military uniforms without markings patrolled the airport, with Ukraine's new leadership accusing them of being sent in by Russia.
Many in Ukraine's south and east are unhappy with the Euromaidan protesters and their goal to bring the Ukraine into a closer alliance with the European Union. They favour a closer relationship with Russia, which now deposed President Viktor Yanukovych had advocated. Yanukovych has since fled to Moscow.
An unidentified armed man patrols a square in front of the International Airport in Simferopol, Ukraine
Writing on Facebook on Friday, Ukraine's Interior Minister Arsen Avakov claimed it was Russian naval forces who had occupied the airports, not protesters acting alone.
They did not attempt to hide this affiliation, he claimed, saying around 120 soldiers had arrived in trucks at Simferopol airport, the regional capital.
He wrote: "I consider what has happened to be an armed invasion and occupation in violation of all international agreements and norms.
"It is a direct provocation... in the territory of a sovereign state."
Russia's Black Sea fleet said its troops have not taken any action at the military airport.
"No units of the Black Sea fleet were deployed in the area of Belbek nor did they take place in blockading it," a statement from the fleet said.
The UK foreign office has updated its travel advice for Ukraine, warning British nationals not to try to leave from Simferopol airport.
It says: "Armed men are reported to have seized Simferopol airport early on 28 February. Although the airport is reportedly operating as usual, we do not advise British nationals to try to leave from there until the situation becomes clearer.
"Sevastopol (Belbek) airport is also reported to be blocked by military and flights are not operating."
Russian flag is raised at the parliament building in Simferopol, Crimea
On Thursday, Ukraine warned Russia that any troop movements outside its Sevastapol base would be seen as an act of aggression.
In Moscow, members of the Just Russia party have submitted a draft law which aims to simplify the joining of new territories to the Russian Federations, and the granting of Russian citizenship to Ukrainians.
"The situation in Ukraine is complicated and unpredictable. It is necessary for Russia to have a maneuver in case the process of Ukraine's breakup continues," one of the party's leaders, Mikhail Yemelyanov, told Itar-Tass, a Russian newspaper.
"The draft laws are ready for submission, but we would like first to consult colleagues from other parties."
Just Russia leader Sergei Mironov told Itar-Tass the new citizenship rule would "pply only to Ukrainian citizens."