Russia's defence ministry said 63 soldiers were killed in the Ukrainian strike on New Year's Eve that destroyed a temporary barracks at a vocational college in Makiivka, near the Russian-occupied regional capital of Donetsk in eastern Ukraine.
Ukraine and some Russian nationalist bloggers have put the Makiivka death toll in the hundreds.
Critics in Moscow lambasted Russian leaders who had been housing the soldiers near an ammunition dump, which the defence ministry said was hit by four rockets fired from US-made Himars launchers.
TV footage showed the huge building reduced to rubble as cranes and bulldozers picked through concrete debris lying over a metre deep.
Kyrylo Budanov, Ukraine's director of military intelligence, refused on Tuesday to claim credit for the attack, but said in an interview that the country's targets would be located “deeper and deeper” behind Russian lines.
Russia hinted at having struck back on Tuesday, with a state media report saying two of the sophisticated Himars had been destroyed in return fire.
“In the course of a counter-battery fight in the area of the city of Kramatorsk, positions were opened and two launchers of the US-made Himars multiple launch rocket system were destroyed, from which shelling of settlements in the Donetsk People’s Republic (DPR) was carried out,” said the report from news agency RIA, citing the Russian defence ministry.
Also in Donetsk, Ukraine's Security Service reported that it had found a makeshift torture chamber with equipment, disposal bags, heavy objects and tasers.
“According to the investigation, representatives of the aggressor country seized houses in the village and illegally held and brutally tortured local residents who refused to co-operate with the enemy,” said the Ukrainian Prosecutor’s Office.
There has been no let up in attacks on Ukraine and officials fear Russia's strategy is to grind down its citizens.
With Russia launching nightly waves of drone attacks on Kyiv and other Ukrainian cities, President Volodymyr Zelenskyy warned attacks were designed to exhaust “our people, our anti-aircraft defences, our energy”.
Ukrainian air force spokesman Yuriy Ihnat told Ukrainian TV that 84 drones had been shot down in two Russian attacks since the new year.
“There is an increase in the use of drones in all directions,” he said.
Mr Ihnat added that Ukraine's armed forces were organising mobile groups to hunt the drones down, using jeeps and other vehicles equipped with anti-aircraft machine guns and searchlights.
Britain's Prime Minister Rishi Sunak on Tuesday spoke to Mr Zelenskyy on the phone to reassure him that further donations of weapons were in the pipeline.
The two men discussed the “abhorrent drone attacks on Ukraine in recent days, and the Prime Minister said the thoughts of the UK were with the Ukrainian people as they continued to live under such bombardment,” Downing Street said in a statement.
“The Prime Minister said Ukraine could count on the UK to continue to support it for the long term, as demonstrated by the recent delivery of more than 1,000 anti-air missiles.
“Work was also under way to provide further equipment in the coming weeks and months to secure Ukraine’s victory on the battlefield”, the Prime Minister added.
“Discussing the Joint Expeditionary Force (JEF) summit in Latvia last month, the Prime Minister thanked the President for joining virtually and said the UK and JEF partners were working closely to provide the vital equipment requested.
“The leaders agreed to stay in close touch in the coming weeks.”