Russia temporarily suspended all flights at the Pulkovo Airport in St. Petersburg on Tuesday, the city government said.
The decision came amid unconfirmed Russian media reports of an unidentified object such as a drone above the city.
The government of Russia's second city said on its official Telegram channel that it had halted all flights at the airport until 12 p.m. local time (9 a.m. GMT). It did not provide a reason for the suspension.
The state-run TASS news agency said that airspace within a 200-kilometer (124-mile) radius of Pulkovo had been closed until 1:20 p.m. local time, citing an unnamed source.
An unconfirmed media report from the online Russian news outlet Baza said an unidentified object had been spotted in the sky and that fighter jets had been dispatched to investigate.
Reuters could not immediately confirm that report.
Data from the Flight Radar website showed a number of domestic flights headed for St Petersburg turning back to their destinations.
Meanwhile, the airspace closure also appeared to be affecting flights en route to the Russian exclave of Kaliningrad, which requires planes to fly over St. Petersburg.
