Russia's aviation watchdog said on Thursday all four Moscow airports had resumed work after the lifting of temporary restrictions that were introduced to ensure the safety of civil aircraft flights. "Aircraft crews,

"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.

The authorities temporarily closed Moscow's four airports, citing safety reasons, but the restrictions were later lifted.

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,’ the statement from Rosaviatsia said.

All four of Moscow's airports including another about 100 miles southwest of the Russian capital were temporarily shut down for undisclosed security concerns, authorities said.