The blasts at sites hundreds of miles from the border between the two countries were the result of a Ukrainian drone attack, Russia's defense ministry said, potentially signaling a new ability by Kyiv to strike far from the front lines of the conflict.
Two aircraft were lightly damaged, the ministry said in a statement, and four people were wounded.
"The Kyiv regime attempted to attack by Soviet-made UAVs the military airfields at Dyagilevo in the Ryazan region and Engels in the Saratov region in an effort to disable Russian long-range aircraft," it said, adding that the unmanned aerial vehicles, or UAVs, were intercepted by air defense while flying at a low altitude.
NBC News has not verified the claims and Ukraine has not taken responsibility. Both airbases are more than 300 miles from the border.
The explosions hit bases involved in launching attacks against Ukraine early Monday just hours before the latest barrage of Russian airstrikes forced residents in the capital, Kyiv, and cities across the country to take shelter as sirens blared.
Power and water outages were felt from Sumy in the northeast to Odesa in the southwest, while at least two people were killed in the southern city of Zaporizhzhia, Kyrylo Tymoshenko, deputy head of the president's office, said in a Telegram post.
Russia's defense ministry said it had launched the strikes "despite the attempts of the Kyiv regime to disrupt the combat work of Russian Long-Range Aviation with a terrorist act."
Moscow has consistently denied targeting civilians, but while such strikes have become an increasingly common feature of the Kremlin’s war, blasts deep inside Russian territory are more unusual.
Ukraine has not taken responsibility, but officials made vague references in social media posts that offered hints the incident could be the work of its forces.
“The Earth is round — discovery made by Galileo. Astronomy was not studied in Kremlin, giving preference to court astrologers. If it was, they would know: if something is launched into other countries’ airspace, sooner or later unknown flying objects will return to departure point,” said presidential adviser Mykhaylo Podolyak in a tweet.
Russian authorities said earlier they were investigating media reports about explosions at the Engels airbase near the city of Saratov in southwestern Russia. 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’s information about a loud explosion and a flash in the early morning in Engels spreading on social networks and the media,” said Saratov Gov. Roman Busargin in a post on Telegram. “There is no reason to worry. Not a single object of civil infrastructure was damaged,” he said.
Separately, a fuel truck exploded at an airfield near the western Russian city of Ryazan, an emergency services spokesperson was quoted as saying by state news agency RIA Novosti. At least three servicemen were killed and three others injured, and a plane was also damaged, they said.
The base houses 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.
Kremlin spokesman Dmitry Peskov said only that President Vladimir Putin “regularly receives information” when asked about the blasts in his daily press call.
“Engels airfield is one of the most important bases of Russian air forces,” said Ukrainian interior ministry adviser Anton Gerashchenko in a tweet. “Two regiments of strategic bombers are stationed here which are part of Russia’s aerial nuclear deterrence tactics."
Moscow has lost over 60-wing aircrafts since the war began on Feb. 24, the U.K. defense ministry said Monday in its intelligence update, adding that aerial missions were down from 300 per day in March to just tens a day.
In a separate tweet, it also said Ukraine had regained control of more than half of its territory that Russia had captured since February.
Analysts said it was unclear that the new wave of Russian strikes were in any way retaliation for the blasts at the airbases.
“They are going to the attack the country as much as possible,” Rajan Menon, a director at Defense Priorities, a Washington-based think tank, told NBC News in a call from Kyiv. “Had it not been for the strike on the base, they would have found other reasons to do so."