Vladimir Putin has accused Ukraine of committing a "terrorist attack" after an overnight drone attack on a school killed at least six and injured 39.
The Russian president said the strike on a school dormitory in the Russian-controlled Luhansk region had also left 15 people missing.
He accused Ukraine of intentionally targeting civilians and said he had asked the Russian defence ministry for options for retaliation.
Photos and videos released by Russian authorities showed rescue workers carrying a man out of the rubble on a stretcher.
Severely damaged buildings could also be seen, one of which appeared to have partially collapsed, as well as fires still burning.
Ukraine has yet to comment. Its forces are fighting to try to recapture Luhansk, one of four regions Russia unilaterally claimed as its own in 2022, in what Kyiv considers an illegal land grab.
Russia's human rights commissioner, Yana Lantratova, said 86 teenagers between the ages of 14 and 18 had been asleep inside the hostel belonging to Luhansk Pedagogical University's Starobilsk school when Ukrainian drones attacked during the night.
Leonid Pasechnik, the top Russian-installed official in Luhansk, said two people had been pulled from the rubble.
Rescue workers were still looking for children trapped beneath the debris, he added.
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Kremlin spokesman Dmitry Peskov said those responsible needed to be brought to justice.
"This is a monstrous crime. An attack on an educational institution where children and young people are present," he said.
"The most important thing now is to take measures to clear the rubble and provide assistance to those who are still trapped beneath it."
Last week, Ukrainian President Volodymyr Zelenskyy promised retribution after laying red roses at the rubble of a Kyiv apartment building where a Russian missile strike had killed 24 people, including three children.
Both sides deny targeting civilians in the war.
On Friday, Mr Zelenskyy said diplomatic efforts to end the war should be reinvigorated. His comments came after he held talks with the leaders of the UK, France and Germany.
In his evening address, the president said he expects proposals from the US on new forms of diplomacy and stressed that the frontline situation was favourable to Ukraine.
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