KIEV, Ukraine -- Ukraine's interior minister accused Moscow's military of blockading an airport near a Russian naval base on Friday and armed men took control of another airport in Ukraine's Crimean capital of Simferopol.
In a Facebook post, Arsen Avakov called the seizure of the Belbek international airport in the Black Sea port of Sevastopol a "military invasion and occupation." He added: "It is a breach of all international agreements and norms."
The Interfax news agency quoted Russian military sources as saying the incident at Belbek airport was intended to stop "fighters" flying in. However, Interfax later quoted a Russian official as saying that no units had approached the airport or blockaded it. NBC News was unable to independently verify either account.
Experts fear tensions stoked by last week's ouster of President Viktor Yanukovych could trigger a deadly international dispute centered on the pro-Russia region of Crimea.
Ukraine's acting president on Thursday 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," Oleksander Turchinov said.
Separately, dozens of armed men in military uniforms without markings were patrolling the main regional airport in Simferopol. They appeared to be allowing the site to function normally.
A eyewitness told Reuters that the men, dressed in military gear and carrying assault rifles, were roving freely in an out of the control tower.
A man called Vladimir, who said he was a volunteer helping the group, said: "I'm with the People's Militia of Crimea. We're here at the airport to maintain order. We'll meet the planes with a nice smile - the airport is working as normal."
Russian news website Lifenews (link in Russian) reported that the armed men were seeking to prevent "radical Ukrainians" from arriving at the airport.
Crimea is the only autonomous region of Ukraine and the only one to have its own constitution and a separate parliament. Most of Crimea’s 2 million inhabitants speak Russian, and describe themselves as Russian - even if they hold Ukrainian passports.
On Thursday, unidentified gunmen also seized the Crimean parliament and raised a Russian flag. The parliament's president also announced plans for a referendum on the region's political future, adding that "Ukraine is sliding towards total chaos, anarchy and economic catastrophe."
Moscow scrambled fighter jets on Thursday, a day after Russian President Vladimir Putin ordered 150,000 troops to be ready for war games near Ukraine. The moves were reminiscent of Cold War brinksmanship.
NATO's secretary-general has urged Russia to avoid taking "any action that can escalate tension."
Ukraine finds itself at the center of a tug-of-war between Russia and the European Union but Secretary of State John Kerry has insisted the country's future is not about “east versus west.”
But separatist sentiment has stoked fears that an east-west split could trigger a civil war.
Wealthy western Ukraine looks toward its European neighbors – especially Poland - and this is where Ukrainian nationalist sentiment is strongest.
In the industrialized east and south, ties are closer to Russia. In some states, most notably Donetsk, Luhansk and Crimea, the outlook is primarily pro-Russian.
Although Moscow says it will not intervene by force, its rhetoric since the removal of pro-Russian Presiden Viktor Yanukovich has echoed the runup to its invasion of Georgia in 2008, when it sent its troops to protect two self-declared independent regions and then recognized them as independent states.
Maria Stromova and Albina Kovalyova reported from Moscow. Jason Cumming reported from London. Reuters and The Associated Press contributed to this report.