Dozens of rifle-toting men, many of them masked, were patrolling the parking lot and entrance of the Simferopol airport, news agencies in the Russian-dominated Ukrainian territory reported.
Ukraine's acting interior minister, Arsen Avakov, accused Russian nationalist militants of attempting "an armed invasion and occupation," Russia's RIA Novosti news agency reported, citing Avakov's Facebook page.
Sevastopol airport duty chief Anatoly Rechenko was quoted by UNIAN news agency as saying armed Russian troops were blocking the roads to the military airport, which was closed and its three charter flights scheduled for Friday canceled.
Gunmen representing themselves as "protectors of the Russian people in the Crimea" also appeared on the border between the Crimean republic and mainland Ukraine, a visitor said.
Crimea has become a flashpoint between Ukraine and Russia since an opposition movement last week forced out President Viktor Yanukovich, who was seen by Ukraine's Russian minority as allied with Moscow and committed to protecting the Russian-speaking community's rights.
Sevastopol airport was not operating, Avakov said. Civilian flights out of Simferopol were operating normally, Western news agencies reported from the scene at Crimea's main transportation hub.
Acting Ukrainian President Oleksandr Turchynov had warned Thursday that Russian military in Crimea should remain inside their bases to avoid an escalation of tensions on the peninsula.
The BBC showed video of the armed men smoking and patrolling the Simferopol airport parking lot as wary travelers went about their business. The British broadcaster showed video of one unidentified gunman saying they were there "to ensure general order" and to prevent "radicals from Kiev" intervening in the Crimean standoff.
Ukraine's UNIAN news agency quoted Simferopol airport spokesman Igor Stratilati as saying the gunmen had left by late morning, although other news agencies said the armed men were still on site.
The Russian minority in eastern Ukraine has reacted with alarm at the sudden turnover of political power in the capital, Kiev. A three-month protest movement in demand of Yanukovich's resignation flared into deadly clashes last week, leaving 82 protesters and police dead and forcing Yanukovich to sign a European Union-brokered agreement to cease fighting and submit to new elections.