Ukrainian drones have struck a college dormitory in the Russian-occupied region of Lugansk, killing six people and wounding dozens of others, Russian officials have said.
As many as 15 others are missing, Russian President Vladimir Putin said in televised comments, describing the incident as a "terrorist" attack.
Mr Putin ordered his defence ministry to prepare a response, without elaborating.
The Lugansk region is in east Ukraine, but is almost entirely occupied by Russia, which claims it as its own.
Images released by the region's Russian-installed governor showed what appeared to be a college in the town of Starobilsk with its windows blown out, flames visible in one of them.
There was no immediate comment from Ukraine.
"Enemy drones attacked the academic building and dormitory of Starobelsk Professional College," Moscow-installed governor Leonid Pasechnik said in a post on Russia's MAX messenger, using the Russian name for the town.
"At the time of the strike, 86 children aged 14 to 18 were there," he added.
Starobilsk lies about 65 kilometres from the front line in east Ukraine.
Russian forces captured the town in 2022, shortly after launching their full-scale offensive.
AFP was not able to immediately verify information about the college that was hit.
'No leniency'
Mr Putin said the attack took place as the students were sleeping.
"At the moment, it is known that six people were killed, 39 were wounded and 15 people are unaccounted for, as the search through the rubble is still ongoing," he said in televised comments.
"There are no military facilities, special services, or related services near the dormitory," he added.
Russia's foreign ministry said that those responsible would "face inevitable and severe punishment".
"There will be no leniency," it said.
Russia's Investigative Committee accused the Ukrainian military of having fired multiple drones at the building.
"As a result of the attack, the five-storey building collapsed to the second floor," it said.
Ukraine, which denies targeting civilians, regularly fires drones at Russian-controlled areas in retaliation for mass Russian strikes on its own people.
The United Nations Human Rights Monitoring Mission in Ukraine has recorded more than 60,000 civilian casualties since 2022, almost 90% of which were in areas controlled by Ukraine.
A massive Russian attack on Ukraine's capital Kyiv last week killed 24 people, including three children, according to Ukrainian authorities.
Russia fired more than 100 drones at Ukraine between late yesterday and this morning, according to the Ukrainian air force.
