Russian soldiers died after lighting a fire beside a pile of ammunition, per Russian outlet 161.ru.
An RPG-7 grenade launcher shell rolled into the fire and exploded, killing at least 12, it said.
The marines lit the fire on their lunch break. Russia is now investigating, per the report.
At least 12 Russian marines were killed when an RPG shell rolled into the fire they had lit beside a pile of ammunition, a source told Russian news outlet 161.ru.
Three sources in Russia's military, law enforcement, and investigative agencies told the outlet about the incident, it said.
One said that about 20 marines lit a fire near the ammunition during their lunch break.
An RPG-7 grenade launcher shell then rolled into the fire and exploded, the source told 161.
The outlet reported that eight marines died immediately, and four more died in hospital a few days later after being seriously wounded.
At least four other soldiers were injured, the report said.
The group were contract soldiers from Vladivostok, a city in southeastern Russia, the report said.
The preliminary cause of the incident was listed as noncompliance with safety regulations, 161 reported.
One of the outlet's sources said Russia's Investigative Committee, which looks into crimes in Russia, is investigating the deaths.
The incident took place on November 3 at the Kuzminsky training ground, close to the Ukrainian border in southern Russia, the report said.
Russia's defense ministry has so far not commented on the incident, according to 161. Business Insider has reached out to the ministry for comment.
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