Dozens of "sleeping" troops were killed by Russian missiles in an attack in southern Ukraine, it is claimed.
A military barracks was targeted in the city of Mykolaiv, with at least 50 bodies pulled from the rubble, it is claimed.
It is feared the final death toll could be higher than 100, with the city's mayor branding the attack "cowardly".
A 22-year-old soldier told AFP news agency: "No fewer than 200 soldiers were sleeping in the barracks.
"At least 50 bodies have been recovered, but we do not know how many others are in the rubble."
A rescue operation is underway to recover any further survivors.
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Mayor Vitaly Kim said: "Yesterday orcs (a nickname for Russian troops) hit our sleeping soldiers with a rocket in a cowardly manner.
"A rescue operation is under way."
Mykolaiv is surrounded by Russian troops, but Ukraine said it has been able to push back invaders.
Earlier this week Mr Kim told Reuters that he and others would stay and fight until the end.
He said: "The size of the Russian army is not infinite. They also lack morale, and their resources are limited.
"The only option they have is to wipe this city off the map and only then they will be able to pass.
"In theory that is possible, but we will try our best not to allow it."
This morning the Ukrainian prosector general's office said that 112 children have been killed so far in the war.
It also said on Telegram that 140 children had been wounded.
The UK's Ministry of Defence (MoD) says Russia "has been forced" to change its operational approach in Ukraine "and is now pursuing a strategy of attrition".
The Kremilin is desperate to hide the high number of Russian casualties in the unprovoked invasion, which has turned the country into a pariah state shunned by the rest of the world.
"This is likely to involve the indiscriminate use of firepower resulting in increased civilian casualties, destruction of Ukrainian infrastructure, and intensify the humanitarian crisis," the ministry said in its latest intelligence update.
"Putin has reinforced his control over Russian domestic media.
"The Kremlin is attempting to control the narrative, detract from operational problems and obscure high Russian casualty numbers from the Russian people."
