Another Russian airfield was set ablaze by a drone strike on December 6, a day after Russia said two of its air bases, including one located hundreds of kilometers from Ukraine, had been hit by drones.
The U.S. State Department said there was no confirmation the strikes were carried out by Ukraine, and Secretary of State Antony Blinken said the United States has "neither encouraged nor enabled the Ukrainians to strike inside of Russia."
He added that it is important to understand what Ukrainians are living through every day with the "ongoing Russian aggression," accusing Russia of "weaponizing winter" through attacks on civilian infrastructure.
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Officials in the Russian city of Kursk, around 90 kilometers north of the Ukraine border, released pictures of black smoke above an airfield, where the governor said an oil storage tank had gone up in flames.
Ukraine did not directly claim responsibility for the strike or two on December 5 that targeted a Russian military air base about 600 kilometers east of Ukraine in the Saratov region and another at an airfield outside Ryazan southeast of Moscow.
Presidential adviser Oleksiy Arestovych said the air base in Saratov is Russia's only base fully equipped for the long-range bombers Russia has used in its campaign to damage Ukraine's energy grid.
A senior Ukrainian official quoted on December 5 anonymously by The New York Times said the drones were launched from Ukrainian territory and at least two planes were destroyed at one of the Russian bases and several more were damaged.
“All this complicates the operation against Ukraine,” Arestovych said. “Yesterday, thanks to their 'unsuccessful smoking,' we achieved a very big result," he said, poking fun at an earlier claim by Russia that explosions at one of its bases had been caused by carelessness with cigarettes.
Russia responded to the December 5 attacks with what it called a "massive strike on Ukraine's military control system," though it did not identify any specific military targets. The Russian missiles destroyed homes and knocked out power as part of the ongoing campaign against Ukraine's civilian infrastructure.
WATCH: A Russian commander stabbed an elderly Ukrainian woman to death during the chaotic retreat from Izyum in September, according to a soldier in his unit.
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Russian Soldier Says Commander Stabbed Ukrainian Woman To Death
Russian Defense Minister Sergei Shoigu said on December 6 that strikes on Ukraine’s energy and transport infrastructure were militarily justified.
The strikes have hit “system of military command, armament plants, and objects connected to them in order to break Ukraine's military potential,” he said.
He claimed that Russian military operations were continuing successfully, and that Moscow's forces had taken a number of settlements in the Donetsk and Luhansk regions while inflicting heavy casualties on the Ukrainian side.
Ukrainian Deputy Minister of Defense Hanna Malyar said heavy battles were taking place in the Donetsk region, "and the price is very high."
"For us, the epicenter of hostilities today is the Donetsk region, and the enemy has concentrated most of its forces there,” she said.
It was the sixth time in memory that Russian forces had tried to push to the borders of the Donetsk region and failed, she added.
"We must understand that when we liberate the temporarily occupied territories, it is a victory. When we stand firm and don't let the enemy in, it's also a victory," Malyar emphasized.
The General Staff of the Armed Forces of Ukraine earlier reported that Russian troops were concentrating their main efforts on conducting offensive operations in the Bakhmut and Avdiyivka areas of the Donetsk region.
Ukrainian forces fought off a fresh round of Russian attacks in the east, Ukraine's General Staff said on December 6, as Russian troops continued their relentless offensive in the Bakhmut and Avdiyivka areas of Donetsk region, and tank and artillery bombardment hit some 20 settlements in the area, including Soledar, Verkhnokamyanske, Andriyivka, and Yakovlyivka.
Ukrainian President Volodymyr Zelenskiy, who visited troops on the front line in the Donbas region, praised soldiers in a selfie video filmed near Bakhmut. He presented medals and shook hands with troops in a hangar.
The president's office said Zelenskiy made the December 6 visit to hand the soldiers state awards on Ukraine's Day of the Armed Forces.
"Today is the day of our gratitude. This is exactly the meaning of December 6 -- and it is forever," Zelenskiy said in his nightly video address. Ukrainians celebrate the Day of the Armed Forces of Ukraine "with words of gratitude, feelings of gratitude, tears of gratitude," he said.