(CNN)About 60 people are feared dead after a Russian aircraft dropped a bomb on a school in Luhansk, eastern Ukraine, where civilians were sheltering on Saturday afternoon, a Ukrainian official said.Serhiy Hayday, head of the Luhansk regional military administration, said the school building was destroyed."The explosion happened inside the building. Rescuers (are) dismantling the debris as quickly as possible. The chances of finding (anyone still) alive are very small. There were 90 people inside the school building; 27 survived, 60 people most likely died," Hayday said.Hayday said a Russian air strike hit the school in the village of Bilohorivka, which is some seven miles from the frontlines of the ongoing conflict in eastern Ukraine, at around 4:37 p.m. (9:37 a.m. ET) on Saturday, causing fires to break out that took almost four hours to extinguish.

Photos shared from the scene showed rescue personnel searching the smoldering ruins of the school building.

Western countries reacted to the bombing with outrage. UK Foreign Secretary Liz Truss said on Sunday she was "horrified," adding: "Deliberate targeting of civilians & civilian infrastructure amounts to war crimes. We will ensure Putin's regime is held accountable."

Russian forces have struck fleeing or sheltering civilians on numerous occasions since Moscow invaded Ukraine.

Schools, nurseries and hospitals have all been targeted by Russia, according to the United Nations.

Russian forces "must face justice for a series of war crimes" committed in the region northwest of Kyiv, following an investigation it conducted in Ukraine, Amnesty International said Friday.

The investigation, based "on dozens of interviews and extensive review of material evidence," has documented "unlawful air strikes on Borodyanka, and extrajudicial executions in other towns and villages including Bucha, Andriivka, Zdvyzhivka and Vorzel."