Reports say that up to 50 armed men briefly entered the main airport of Simferopol, the capital of the Crimean region, as Ukraine plunges deeper into crisis.
The armed men carrying Russian navy flags entered the airport overnight after arriving in lorries, the reports said.
One witness told AFP news agency that armed men appeared to be in control of the airport early on Friday morning.
The incident followed a day of dramatic developments with pro-Russian armed men taking over government buildings in Simferopol and raising a Russian flag.
Ukraine warned Russia on Thursday against "military aggression" amid reports that Viktor Yanukovich, the deposed Ukrainian president, was being sheltered by Russia.
|Western diplomats call for cool heads over Ukraine
Oleksander Turchynov, acting Ukraine president, told parliament he had ordered troops and police to take "all measures" needed to protect Ukranian citizens after the apparent pro-Russian takeover of buildings.
Turchynov's statement also gave warning that any movement by Russian military forces in the Crimean Black Sea port of Sevastopol, would be considered a "military aggression".
Pro- and anti-Russian protesters clashed on Wednesday in the region, where the majority of the population is ethnic Russian.
Arsen Avakov, Ukraine interior minister, said 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".
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The area has been closed 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.
Chuck Hagel, US defence secretary, warned of escalation of tension between Ukraine and Russia.
Speaking at a NATO meeting in Brussels, he said that the US was watching Russian military movements closely, and urged the country not to take any action on Ukraine that could be misinterpreted "at a time of great tension".
David Cameron, UK prime minister, also asked Russia on Thursday to respect Ukraine's sovereignty.
"Every country should respect the territorial integrity and sovereignty of Ukraine. Russia has made that commitment and it's important that Russia keeps its word," he said.
For his part, John Kerry, US secretary of state, said he had received assurances from his counterpart Sergei Lavrov, who "reaffirmed President Putin's statement that Russia will respect the territorial integrity of Ukraine.
"We believe that everybody now needs to step back and avoid any kind of provocation."
Reports also began to emerge on Thursday that Yanukovich was now in Russia.
Breaking his five-day silence, he insisted that he was still the head of state, and called the new leaders of Ukraine "illegitimiate", Russian news organisations reported.
|Pro-Russian gunmen occupy Crimea
The autonomous government of Crimea still considers Yanukovich to be Ukraine's president.
Vladimir Konstantinov, speaker of Crimea's parliament, said on Thursday the autonomous republic appointed a new prime minister, Sergei Aksenov, with Yanukovich's approval.
Al Jazeera's Rory Challands, reporting from Moscow, reported that it was almost certain that Yanukovich was in Russia.
Russia's state news agency has said that Yanukovich will hold a news conference on Friday in the Russian city of Rostov-on-Don.
Ukraine's interim government has issued an arrest warrant for Yanukovich, accusing him of being responsible for the deaths of at least 70 protesters on February 21 protests in Kiev.
The parliament had voted on Tuesday to send Yanukovich, if captured, to stand trial for "serious crimes" at the International Criminal Court.
Source: Al Jazeera and agencies