Blasts were heard at Russia's Engels air base, hundreds of miles from the frontlines in Ukraine, Ukrainian and Russian media outlets reported early on Monday.

There was no immediate official confirmation of the blasts and Reuters was not able to independently verify the reports.

The RBC-Ukraine news agency reported that two explosions took place.

The Russian news outlet Baza reported, citing local residents, that air raid sirens were wailing and an explosion was heard.

The air base, near the city of Saratov, about 450 miles southeast of Moscow, was hit on Dec. 5 in what Russia said was Ukrainian drone attacks on two Russian air bases that day.

The twin strikes dealt Moscow a major reputational blow and raised questions about why its defenses failed, analysts said, as attention turned to the use of drones in the war between neighbors.

Ukraine has never publicly claimed responsibility for attacks inside Russia, but has said, however, that such incidents are "karma" for Russia's invasion.