Authorities have closed the skies over Russia’s second-largest city St. Petersburg until 12 p.m. local time, local officials said Tuesday.

St. Petersburg’s Pulkovo International Airport has halted flight departures and arrivals. According to the Flight24 flight-tracking website, no civilian aircraft are in the sky over St. Petersburg or the surrounding Leningrad region.

The Baza and 112 Telegram channels reported that air defense officials activated the “Cover” signal, which is used when unidentified aircraft and other objects are detected in the airspace.

The signal requires the immediate landing or withdrawal of all aircraft in the area, with the exception of aircraft used to combat violators and perform search and rescue tasks.