Both sides of the war between Russia and Ukraine are taking hits, as explosions rocked two Russian air bases on Monday, and multiple Ukrainian cities sustained damage from missiles, the Wall Street Journal reported.

One explosion was at a Russian base near Engels, a city in the Russian region of Saratov, with regional Governor Roman Busargin saying authorities were looking into the matter, according to the Journal. The other explosion occurred when a fuel tanker blew up at an airfield in Ryazan.

No civilians were hurt in the Engels blast, Busargin said, while Russian state-run news agency RIA Novosti said at least three people were killed in the Ryazan explosion.

Ukraine has not taken responsibility for the explosions, but an aide to Ukrainian President Volodymyr Zelenskyy teased involvement.

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"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" adviser Mykhailo Podolyak tweeted.

Ukrainian officials have also reportedly alluded to an ability to hit targets deeper into Russia.

In Ukraine, people flooded into bomb shelters – including ones set up in the underground transit system – as Russia rained missiles down into the country, According to a Zelenskyy aide, two people died in the blasts, at least two others were injured and several homes belonging to civilians in the southeastern region of Zaporishshia were wrecked.

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Bomb shelter in Ukrainian metro system

People rest in the subway station being used as a bomb shelter during a rocket attack in Kyiv, Ukraine, Monday, Dec. 5, 2022. Ukraine’s air force said it shot down more than 60 of about 70 missiles that Russia fired on in its latest barrage against Ukraine. It was the latest onslaught as part of Moscow’s new, stepped-up campaign that has largely targeted Ukrainian infrastructure and disrupted supplies of power, water and heat in the country as winter looms. (AP Photo/Andrew Kravchenko)

In addition to those strikes, blasts were heard in Ukraine's capital city of Kyiv and the governor of the southern Mykolaiv region said he saw cruise missiles above.

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Andriy Yermak, Ukraine’s chief of presidential staff, reportedly vowed in a Telegram post that his country would respond.

"Do not ignore the alarm, we will fight back," he said.