Ukrainian and Russian media outlets said early on Monday that explosions were heard at Russia's Engels air base, hundreds of kilometers from the frontlines in Ukraine.

There was no official confirmation of the explosions immediately, and Reuters could not independently validate the reports.

According to the RBC-Ukraine news agency, there were two explosions.

Baza, quoting residents, stated that air raid sirens sounded, and an explosion could be heard.

The air facility near Saratov, approximately 730 kilometers (450 miles) southeast of Moscow, was attacked on December 5 in what Russia claimed were Ukrainian drone assaults against two Russian air facilities on that day.

Analysts said the twin strikes severely damaged Moscow's prestige and raised concerns about why its defenses failed as the focus shifted to using drones in the conflict between the two countries.

Ukraine has never openly claimed responsibility for assaults within Russia but has stated that such occurrences are "retribution" for Russia's invasion.