Russian President Vladimir Putin ordered his military to prepare options for retaliation against Ukraine on Friday after Moscow accused Kyiv of what it described as a deliberate drone strike on a college in the town of Starobilsk.
Ukraine's military denied responsibility for the attack, saying it had struck an elite drone command unit in the area and that its forces complied with international humanitarian law.
Putin said there were no military facilities in the area. Reuters was not able to independently verify what happened.
At the scene on Saturday, a crane was working to remove rubble from a yawning gap in the building.
Inside one shattered classroom, bricks and dust covered rows of student desks with "I love English" written on the wall. Elsewhere, a stairwell was blocked by debris.
Russia's state-run news agency RIA reported the death toll had risen to 18, citing the emergency ministry.
Three people remained trapped under the rubble.
Leonid Pasechnik, head of the Russian-installed administration in the region, published a preliminary list containing details of 11 victims, most of them 19-year-old women.
A resident said rockets had targeted a former base and drones had then hit the student dorm, causing fires to break out.
At an emergency meeting of the UN Security Council called by Russia on Friday, Russia accused Ukraine of war crimes over the incident.
Ukraine said it was a baseless claim that had not been independently verified.
Several countries called for access to the site, while UN officials decried all attacks on civilians, recalling a Russian missile attack on a UN warehouse in Ukraine this week that had killed two workers and destroyed $US1 million ($A1.4 million) worth of aid.
Thousands of Ukrainians have been killed in air strikes far from the largely static front line across the southeast of the country, around a fifth of which is controlled by Russian forces.
Russia has targeted Ukraine's power supplies and infrastructure, while Ukraine has stepped up attacks on oil facilities inside Russia this year, sometimes resulting in casualties. Both sides deny targeting civilians.
Falling debris from drones triggered a fire at an oil terminal in Russia's Black Sea port of Novorossiysk and two people were injured, Russian officials said early on Saturday.
Ukraine's military said it had hit Russia's Sheskharis Black Sea oil terminal in Novorossiysk and nearby Grushova oil depot.
President Volodymyr Zelenskiy said the military had also hit a large chemical plant in Russia's Perm region.
Perm regional governor Dmitry Makhonin said earlier that an industrial facility, which he did not name, had been targeted by Ukrainian drones but that they had been shot down and caused no damage.
