According to local officials and state media, explosions shook two Russian military airbases on Monday hundreds of miles from the Ukrainian border.

The mysterious explosions, which were reported at bases used to launch attacks against Ukraine, occurred just hours before the most recent barrage of Russian airstrikes, which forced citizens of the nation’s capital, Kyiv, and other cities to seek shelter as sirens wailed.

According to Kyrylo Tymoshenko, the deputy head of the president’s office, there were power and water outages from Sumy in the northeast to Odesa in the southwest, and at least two people died in the southern city of Zaporizhzhia.

Although such attacks have become a more frequent aspect of the Kremlin’s war, blasts deep inside Russia are more unusual. Moscow has consistently denied targeting civilians.

Local media reported that there were several explosions early on Monday at the Engels airbase in southwest Russia, close to the city of Saratov. Residents were said to have heard a loud noise.

According to Russian authorities, they are looking into media reports about explosions at the base. According to the Associated Press, it is home to the nuclear-capable Tu-95 and Tu-160 strategic bombers that have been used in attacks against Ukraine.

Saratov governor Roman Busargin wrote on Telegram, “Information about a loud explosion and a flash in the early morning in Engels is spreading on social networks and in the media. 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 close to Ryazan, in western Russia, according to a spokesperson for the emergency services, as reported by the official news agency RIA Novosti. According to them, a plane was also damaged and at least three servicemen were killed, and three others were hurt.

According to the AP, the base is home to long-range flight tankers that are used to refuel bombers while they are in the air.

The cause of the explosions remained unknown; when asked about them during his daily press conference, Kremlin spokesman Dmitry Peskov only said that President Vladimir Putin “regularly receives information.”

Although Ukraine has not admitted guilt, officials made ambiguous allusions in social media posts that suggested the incident might portend a new capability for Kyiv to strike deeply inside Russia. The distance between the two airfields and the border is more than 300 miles.

Galileo made the discovery that the Earth is spherical. In the Kremlin, astronomy was not studied, giving court astrologers the upper hand. If it were, they would be aware that if something is launched into another country’s airspace, it will eventually return to its original location, according to presidential adviser Mykhaylo Podolyak.

According to Anton Gerashchenko, a consultant for the Ukrainian interior ministry, Engels Airfield is one of the most significant bases for the Russian air force. “Two regiments of strategic bombers are stationed here which are part of Russia’s aerial nuclear deterrence tactics,” he said.

The U.K. defense ministry reported Monday in its intelligence update that Moscow has lost over 60 wing aircraft since the war started on February 24. It also noted that daily aerial missions had decreased from 300 in March to just ten.

It added that Ukraine had reclaimed control of more than half of the territory that Russia had taken since February in a separate tweet.

Analysts opined that it was unclear whether the latest round of Russian attacks was in any way retaliation for the airbase explosions.

“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,” he said.