Dozens of Ukrainians are feared dead after a Russian bomb flattened a school sheltering about 90 people in its basement.
A bombing raid on a Ukrainian school where people were sheltering is a possible war crime, Foreign Secretary Liz Truss has said.
The governor of Luhansk province, one of two areas that make up the eastern industrial heartland known as the Donbas, said the school in the village of Bilohorivka caught fire after Saturday's bombing.
Emergency crews found two bodies and rescued 30 people, Governor Serhiy Haidai said.
"Most likely, all 60 people who remain under the rubble are now dead," the governor wrote on the Telegram messaging app.
The Foreign Secretary said she was "horrified" and added Vladimir Putin's regime in Moscow would be held to account.
Ms Truss tweeted: "Horrified by Russia’s latest attack on a school in Luhansk, resulting in the deaths of innocent people sheltering from Russian bombardment".
She said the deliberate targeting of civilians and infrastructure "amounts to war crimes" and "we will ensure Putin's regime is held accountable".
Her comments came as G7 leaders, including Boris Johnson and Joe Biden, prepared for talks with Ukraine's president Volodymyr Zelensky.
The Ministry of Defence (MOD) has suggested that Russian forces were struggling because of the high casualty rate among Moscow's commanders.
A defence intelligence update said: "Difficulties in command and control, as well as faltering Russian performance on the frontline, have drawn senior commanders onto the battlefield, likely to take personal leadership of operations."
This has in turn put them at risk of being targeted, with "disproportionately high losses of Russian officers in this conflict".
The update added: "This has resulted in a force that is slow to respond to setbacks and unable to alter its approach on the battlefield."