The Flight Radar flight tracking website on Tuesday reported that a number of Russian domestic flights headed for St. Petersburg were turned back to their departure points.

Russia's Defense Ministry later said it had carried out air drills involving interceptor jets, which were not previously announced and caused the cancellation of flights.

Flights halted, then resumed

At least five flights from Moscow to Russia's second city were diverted back to the capital as of 11 a.m. local time (0800 UTC/GMT), after initially circling in the air.

Traffic was restarted shortly afterward, amid unconfirmed Russian media reports that an unidentified object such as a drone had been spotted in the area.

Online Russian news outlet Baza said jet fighters had been dispatched to investigate after the unidentified object was spotted in the sky. Reports later said the jets had not found anything.

News agencies reported later on Tuesday that flights had started up again and that a temporary airspace ban within a 200-kilometer (124-mile) radius of Pulkovo Airport had been lifted by noon local time.

In a statement issued about an hour after flights had restarted, the Defense Ministry said it had been carrying out exercises that involved the dispatch of fighter jets in Russia's western air space.

"During the training, air defense forces worked on the detection, interception, and identification of targets, as well as interacting with emergency services and law enforcement agencies," Russian news agencies cited the ministry as saying.

Pulkovo, some 15 kilometers (about 9 miles) south of central St. Petersburg, is Russia's fourth busiest airport. The airspace closure was also said to have affected flights on the way to and from the Russian exclave of Kaliningrad, which requires plans to fly over St. Petersburg because nearby countries have closed their airspace to Russian aircraft.

In a media briefing, Kremlin spokesman Dmitry Peskov did not comment on the cause of the flight disruption, but said President Vladimir Putin had been kept fully informed of the situation.

rc/nm (Reuters, Interfax)