Ukraine's interior minister says Russian forces have taken control of two airports in the Crimea region and condemned them as an armed invasion and occupation.
Arsen Avakov said there had been no bloodshed or clashes when armed men he described as Russian naval forces took over a military airport near the port of Sevastopol where the Russian Black Sea fleet has a base.
Reuters reports armed men he described as representing the Russian Federation had taken over Simferopol international airport without clashes.
Mr Avakov called for talks and said it was an armed invasion and occupation in violation of all international agreements and norms.
Earlier, Crimea's regional parliament, which remains under the control of pro-Moscow gunmen, voted to hold a referendum in May to expand the region's autonomy from Kiev and replace the local government with a pro-Russian official.
The raid followed the ousting of Kremlin-allied Ukraine president Viktor Yanukovych last weekend in an avalanche of change following the deaths of scores of people in clashes in Kiev.
Russia warned on Crimea escalation
Acting Ukrainian president Olexander Turchynov has warned that any movement by Russian troops outside their naval base in Crimea will be seen as an act of aggression.
Armed men have seized the regional assembly building in the Crimean capital, Simferopol, as pro-Russian and pro-Ukraine groups face off in the autonomous region and Russia has put 150,000 troops on high alert on its border with Ukraine.
Acting president Olexander Turchynov was speaking in Kiev after Russian flags were raised at the Crimean parliament.
Moscow said it was willing to work with the West on averting a crisis, but warned foreign powers against taking decisions on behalf of Ukrainians, the BBC reports.
Unidentified armed men entered the Crimean parliament by force on Thursday morning and were cheered by a handful of pro-Russian demonstrators who gathered round the building, despite a police cordon.
Western leaders were quick to urge Moscow and Crimean activists not to escalate tensions further.
Nato Secretary General Anders Fogh Rasmussen said he was "concerned about developments in Crimea" and urged Russia "not to take any action that can escalate tension".
US Defence Secretary Chuck Hagel told Russia "not to take any steps that could be misinterpreted, or lead to miscalculation, during a very delicate time".
Both British Prime Minister David Cameron and German Chancellor Angela Merkel voiced their concerns at a joint news conference in London.
The warnings came as Russia performed a second day of military exercises.
Russian President Vladimir Putin ordered a snap drill on Wednesday to test the combat readiness of troops in central and western Russia, near the border with Ukraine.
Russian Foreign Minister Sergei Lavrov said he had spoken to US Secretary of State John Kerry, who proposed co-operation to resolve the crisis in Ukraine.
But he stressed the need to implement an EU-brokered peace deal agreed between Mr Yanukovych and opposition parties before his departure from office last week.
Amid the rising tensions, the Crimean parliament announced it would hold a referendum on expanding the region's autonomy on 25 May.
The uncertainty in Ukraine has sent its currency, the hryvnia, tumbling to a record low.
New PM Arseniy Yatsenyuk accused ousted former president Victor Yanukovych and his government of stripping the state coffers bare, telling parliament billions of dollars had been transferred to offshore accounts in the past three years.
The International Monetary Fund said it had received a request for assistance from the new government and would be sending a team to Kiev in the coming days.