The military airport in Sevastopol, was seized on Friday by what appear to be Russian servicemen.
Armed men who took control of the airport in Ukraine's Crimean regional center of Simferopol on Friday and have been patrolling the airport grounds but had not moved into the passenger terminal and appeared to be allowing the airport to function normally.
A Reuters eyewitness at the scene said the men, dressed in full battle gear and carrying assault rifles and machine guns, were moving freely in an out of the control tower.
A man, who said he was a volunteer helping the group, said: "I'm with the People's Militia of Crimea. We're simple people, volunteers ... We're here at the airport to maintain order. We'll meet the planes with a nice smile - the airport is working as normal."
He added that though he and other volunteers supported the actions of the armed men they did not know where the group was from.
A day before, unidentified gunmen seized the Crimean parliament and raised a Russian flag. The gunmen issued no demands and police hardly seemed to treat the event as a major security incident.
Oleksander Turchinov, Ukraine's acting president, warned Russia not to move personnel beyond areas permitted by treaty for those using its naval base: "Any military movements, the more so if they are with weapons, beyond the boundaries of this territory will be seen by us as military aggression," he said.
The regional prime minister said on Thursday he had spoken to the group by telephone, but they had not made any demands or said why they were there. They had promised to call him back but had not done so, he said.
Interfax news agency quoted eyewitnesses as saying that about 50 armed men, dressed in uniforms that bore no designated marks, arrived at the airport in three trucks and took it over.
The agency said separately that a group of people had headed towards the airport, some of them carrying the ensign of the Russian navy. Part of Russia's Black Sea fleet is based in Sevastopol in Crimea.
Late on Friday morning it was reported by Interfax that Russian servicemen wearing helmets and armoured body protection and backed by armoured personnel carriers had taken up position on the perimeter of the airport at Sevastopol.
The agency quoted military sources in the region as saying that they had gone to Belbek military airport to stop "fighters" flying in.
Ukraine's interior minister said on Friday, "I consider what has happened to be an armed invasion and occupation in violation of all international agreements and norms," Interior Minister Arsen Avakov said on his Facebook page, describing it as an armed provocation and calling for talks.