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
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A building burned from a strike as Russia's attack on Ukraine continues, in Bakhmut. Reuters -
Antonina, a refugee from Bakhmut in Donetsk region, with dogs she rescued in Izyum, Kharkiv. AFP -
A resident gives her neighbours hot food brought by volunteers in Izyum. AFP -
A Ukrainian soldier salutes as he works to build a bunker with sand in Bakhmut. Reuters -
Volodymyr Kovalov, 77, carries tree branches attached to his bike as he collects wood for heating and cooking in Kherson region. AFP -
A local resident takes pictures of a destroyed monastery in Dolyna, eastern Ukraine. AFP -
A decoy dummy made with the clothes of a Russian soldier at the entrance of a destroyed cinema in Kamyanka, eastern Ukraine. AFP -
Destroyed houses in the city of Kamyanka. AFP -
Caesar, 50, a Russian who joined the Freedom of Russia Legion to fight on the side of Ukraine, stands in front of a destroyed monastery in Dolyna. AFP -
An employee stands next to a shelter at a stainless pipes plant in Nikopol, Dnipropetrovsk region. AFP -
Ivan and Iryna Kalinin before the start of the Russian invasion of Ukraine. Iryna, who was pregnant, was killed in a Russian air strike on Mariupol's maternity hospital. Ivan returned to the occupied city to rebury his wife and baby. AP -
Natalia, 67, sits inside her house that was damaged in a Russian military strike, in Kramatorsk, Ukraine. Reuters -
Ukrainian soldiers ride in a military vehicle in Bakhmut, a city in the Donetsk enclave. AP -
Smoke billows from a building on fire after a Russian attack in Bakhmut. AP -
Sasha and her grandfather stand outside their home in Bakhmut. AP -
Ukrainian soldiers set up a barricade in Bakhmut. AP -
Nastya carries her cat as her mother Anna says goodbye to neighbours with whom they lived for months in a basement during Russian attacks in Soledar, a city in the Donetsk enclave of Ukraine. AP -
Anna and Nastya in the basement during a Russian attack nearby. AP -
A Ukrainian soldier is wheeled on a stretcher at a hospital in the Donetsk enclave. AP -
A woman looks a building damaged by Russian shelling in Kherson. AFP -
Oleksandra Koshkina, 85, sits in a car as she returns to her village of Torske, in the Donetsk enclave. Reuters -
A resident of Bakhmut walks along an empty street. Reuters -
A street market in Bakhmut. Reuters -
A Ukrainian soldier poses for a picture in the Donetsk enclave. Reuters -
Officials tend to a patient on a plane carrying Ukrainian Jewish refugees as they wait to disembark at Israel's Ben Gurion Airport near Tel Aviv. AFP
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.
Russia, along with fellow permanent members Britain, France, the US and China, can veto any resolution in the council.
