Russian authorities on Thursday closed all four of Moscow's airports, plus a fifth one in a city about 100 miles southwest of the country's capital, citing unspecified safety concerns.

"To ensure the safety of civil aircraft flights, temporary restrictions have been introduced on the operation of Sheremetyevo, Domodedovo, Vnukovo, Zhukovsky and Kaluga airports," Rosaviatsiya said on Dec.

All four of Moscow’s airports have been temporarily closed without any reason given, according to Russia’s aviation watchdog. ‘Aircraft crews, air traffic controllers and airport services are taking all necessary measures to ensure flight safety,