The accident happened overnight on Lake Syamozero, close to the border with Finland, according to Vladimir Markin, a spokesman for Russia's investigative committee.
Updating an earlier toll of 12 dead, an source in the local emergency services told TASS news agency that ‘two more bodies of adolescents have been found’, taking the number of drowned to 14.
The children, who were aged between 12 and 15, were ‘apparently not wearing life jackets’, Russia's children's rights watchdog, Pavel Astakhov, told the RIA Novosti agency.
The group was taking part in a summer camp close to the lake,
The emergencies ministry said that the children were sailing on two boats and a raft when they got caught in a storm after nightfall.
Rescuers saved more than 30 people but were still searching for four others, the ministry told RIA Novosti.
Moscow mayor Sergei Sobyanin wrote on Twitter Sunday that 10 of the dead children were from Moscow.
Investigators have opened a criminal probe into the incident to determine whether water safety rules were followed.
An instructor at the summer camp has been detained for questioning, investigators said.