Two Russian military airbases were hit hundreds of miles from the border with Ukraine on Monday, according to local officials and state media.
The explosions were reported at bases involved in launching attacks against Ukraine just hours before the latest barrage of Russian air strikes forced residents in the capital, Kyiv, and cities across the country to take shelter as sirens blared.
In a Telegram post, Kyrylo Tymoshenko, deputy head of the president's office, said power and water shortages were felt from Sumy northeast to Odesa southwest, while at least two people were killed in the southern city of Zaporizhzhia.
Moscow has denied targeting civilians, but bombs deep inside Russia are more unusual, despite the fact that such strikes have become increasingly common feature of the Kremlin's war.
Multiple explosions occurred early Monday at the Engels airbase near the city of Saratov in southwestern Russia, with residents quoted as hearing a loud sound.
Russian authorities said they were investigating the media reports about the explosions on the base. It houses the nuclear-capable Tu-95 and Tu-160 strategic bombers that have been involved in carrying out strikes against Ukraine, according to the Associated Press.
There is information on a flash in the early morning in Engels spreading across social networks and the media, said Saratov governor Roman Busargin in a post on Telegram. He said that not a single object of the civil infrastructure was damaged.
A fuel truck exploded at an airfield near the western Russian city of Ryazan, an emergency services spokesman was quoted as saying by state news agency RIA Novosti. At least three servicemen were killed and three others injured, and a plane was damaged, they said.
The base has long-range flight tankers that serve to refuel bombers in the air, according to the AP.
Ryazan officials have not commented on the blast and NBC News has not verified the reports.
The origin of the explosions remained unknown, with Kremlin spokesman Dmitry Peskov saying only that President Vladimir Putin gets information when asked about the blasts in his daily press call.
While Ukraine has not taken responsibility, officials made vague references in social media posts that offered hints that the incident could signal a new ability for Kyiv to strike deep inside Russia. Both airbases are located more than 300 miles from the border.
The discovery of the Earth was made by Galileo. Astronomy was not studied in Kremlin, giving preference to court astrologers. If something was launched into other countries airspace, sooner or later unknown flying objects will return to the departure point, presidential adviser Mykhaylo Podolyak said in a tweet.
The airfield of Engels is one of the most important bases of Russian air forces, said Anton Gerashchenko, Ukrainian interior ministry adviser. Two regiments of strategic bombers are stationed here, part of Russia's aerial nuclear deterrence tactics, he said.
Moscow has lost more than 60 wing aircraft since the war began on February 24, according to the U.K. defense ministry's intelligence update. It said in March that aerial missions in March were down from 300 per day to just tens a day.
In a separate tweet, it said Ukraine had regained control of more than half of the territory that Russia had captured since February.
Analysts said it was not clear whether the new wave of Russian strikes was in any way retaliation for the blasts at the airbases.
Rajan Menon, a director at Defense Priorities, told NBC News in a call from Kyiv that they are going to attack the country as much as possible. Had it not been for the strike on the base, they would have found other reasons to do so, he said.