Pro-Kremlin militants on Tuesday ordered a suspension of all flights in or out of Crimea's main airport in Simferopol except those connecting to Moscow, an AFP reporter on the scene said.
Militants have taken over air traffic control at the airport and a flight that took off from Kiev on Tuesday was forced to turn back to the capital after being refused permission to land.
A militia member, Ivan, who declined to give his surname, told AFP: "Air traffic control has been taken over, as well as the runway."
He said the move was aimed at preventing activists from Kiev from coming to Crimea.
Ukraine International Airlines said on its website that flights between Kiev and Simferopol on Tuesday and Wednesday had been cancelled "due to airspace restrictions".
A source who works in the airport's air traffic control centre told AFP armed men had entered and forced out local workers on Monday night. It was unclear if and when they may be allowed to return, the source said.
Pro-Moscow authorities who have seized power in the Black Sea peninsula and are seeking to join with Russia have taken a number of steps in recent days to isolate the region from the rest of Ukraine and control entry points.
An AFP reporter at the airport said all flights to and from Kiev or Istanbul had been cancelled but that two planes had arrived from Moscow and another was scheduled to land.
Lenura Settarova, 38, had been trying to flee Simferopol with her daughter to stay with friends and family in Istanbul until their flight was cancelled.
"It's dangerous here," Settarova said, adding that they feared "war will start" and "bombings and shootings" in the city.
"It's not safe in the city. I'm a student, my classes have stopped and I'm not allowed to go outside," said her daughter, Emina, 20.
An employee of Ukraine International Airlines, who gave her name only as Tatyana, said she did not know why the flights had been cancelled or when they might resume.
"All I know is that it's because of an instruction from air traffic control," she said.
Flights from Kiev to Simferopol were previously cancelled on February 28 -- a day after armed men seized control of parliament and installed a pro-Moscow government in Crimea -- but resumed within less than 48 hours.