The two main airports in the Ukrainian region of Crimea have been taken over by armed men in Russian uniform, Ukraine's interior minister has said, BBC reported.
The men in Sevastopol airport are Russian military, Arsen Avakov said.
It is not immediately clear who the men in Simferopol airport are.
Relations between Russia and the Ukraine have been strained since the ousting of Ukrainian President Viktor Yanokovych, who is now in Russia.
On Thursday, pro-Russian armed men stormed the parliament in Crimea, Ukraine's only Russian-majority region.
Witnesses told Interfax-Ukraine news agency that around 50 men had arrived at Simferopol airport carrying Russian navy flags.
The airport is said to be operating as normal.
A man called Vladimir told Reuters he was a volunteer helping the group, though he said he did not know where they came from.
"I'm with the People's Militia of Crimea. We're simple people, volunteers," he said.
"We're here at the airport to maintain order. We'll meet the planes with a nice smile - the airport is working as normal."
On Thursday, a separate group of unidentified armed men entered Crimea's parliament building by force, and hoisted a Russian flag on the roof.
The Crimean parliament later announced it would hold a referendum on expanding the region's autonomy on 25 May.
Recent developments in the Crimea region - which traditionally leans towards Moscow - heightened tensions with Russia, which scrambled fighter jets to monitor its borders on Thursday.
US Secretary of State John Kerry called on all sides to "step back and avoid any kind of provocations" on Thursday.
The US has sought assurances from Russia after President Vladimir Putin ordered snap military drills to test the combat readiness of troops in central and western Russia, near the border with Ukraine earlier in the week.
Mr Kerry said he had spoken to his Russian counterpart, Sergei Lavrov, who vowed to respect Ukraine's "territorial integrity".
Crimea - where ethnic Russians are in a majority - was transferred from Russia to Ukraine in 1954.
Russia, along with the US, UK and France, pledged to uphold the territorial integrity of Ukraine in a memorandum signed in 1994.