As many as 15 others remained missing as of Friday afternoon, Russian President Vladimir Putin said in televised comments, describing the incident as a "terrorist" attack.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."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.Ukraine's General Staff said on Friday it had hit a Russian military unit's headquarters around Starobilsk, denying it had targeted civilians and called Moscow's allegations "manipulative information".It said its forces had struck "one of the headquarters of the 'Rubikon' unit in the area of the city of Starobilsk", adding that the unit was involved in carrying out strikes on Ukrainian civilians."The Armed Forces of Ukraine carry out strikes on military infrastructure and facilities used for military purposes, strictly adhering to the norms of international humanitarian law, and the laws and customs of war," it added in a statement.Starobilsk lies about 65 kilometres (40 miles) 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'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.