Three soldiers were reported dead on Monday in a drone explosion at a Russian air base.
The troops were killed by falling debris after a Ukrainian drone was shot down, pro-Kremlin media said, citing the Russian Defence Ministry.
The blast at the Engels base in Russia, more than 600 kilometres from Ukrainian-controlled territory, was the second in a matter of weeks.
Ukraine has neither confirmed nor denied involvement in such explosions behind Russian lines.
“If Russians thought that war won’t touch them far behind the front lines, they were deeply wrong,” Ukrainian air force spokesman Yuriy Ihnat said on television, without claiming responsibility.
The Engels base is home to heavy bombers that are part of Russia's nuclear deterrent.
Russia said no planes were damaged by the latest explosion, after two were reported hit in the incident on December 5.
A fuel tank exploded the same day at the Dyagilevo airfield, Russian officials said, with three people killed.
Russian bombers have been used to fire cruise missiles at Ukraine, it is believed, in a barrage of attacks that have left millions without power.
Ukraine conflict — in pictures
Western officials say Russia has also used Iranian-made drones, in an arrangement hailed by Tehran as proof of its military firepower.
Ukrainian President Volodymyr Zelenskyy said in an overnight address that Russia's aerial attacks were meant to “compensate for its losses”.
“There are only a few days left this year. We must be aware that our enemy will try to make this time dark and difficult for us,” Mr Zelenskyy said.
“I know that the darkness will not prevent us from leading the occupiers to new defeats. But we have to be ready for any scenario.”
Ukraine's Foreign Ministry Dmytro Kuleba said a call would be made on Monday to expel Russia from its permanent seat on the UN Security Council.