Two people were seriously injured when a Ka-52 helicopter suddenly struck at an enclosure for spectators at Luzhsky military field near St Petersburg.
A missile is seen hitting the area close to where a man filming the exercise was standing.
Unconfirmed reports said that the wounded were journalists covering the major war games which have provoked massive concern in the West.
The Ka-52 Alligator helicopter is the newest intelligence and attack helicopter able to attack armoured vehicles and people.
An informed source told 66.ru: “They were approaching the targets, had some 500 metres left to fly. According to their task on the route to target they should switch on the electric chain of missiles.”
The unnamed source said the rockets were fired early as a result of a short circuit.
“Something went wrong, the missiles were immediately launched,” he added. “The parts of the missiles hit a large area, at least two cars were burned, two people wounded, they are in the hospital now.
“Most likely, the victims were journalists.”
Journalists from Russia, Belarus and many other countries – including the West – have been accredited for the Zapad war games.
According to a report by Russia’s Western Military District press office on 17 September the specific exercise at Luzhsky involved Mi-28H and Ka-52 helicopters performing training missions involving air intelligence and attacks on ground targets.
They were meant to be “covering troops from the air”.
President Putin‘s spokesman Dmitry Peskov said: “This question should be addressed to the ministry of defence and not to the president’s administration.”
A statement by the Western Military District said there were “no incidents related to army aviation” on 18 September. It implied the incident was at a different place and time.
The 66.ru report said the incident could have been on 18 or 19 September.
The clip shows two helicopters approaching a row of cars and spectators. One of the helicopters suddenly appears to fire on the ground towards the person holding the camera, who starts running.
The video was captioned as “supplied by a 66.ru reader”.
Attempts were made to remove the video from the internet.
St Petersburg-based website Fontanka.ru said it was aware that military investigators were looking into the incident and are considering launching a criminal case on suspected violations of safety rules governing the use of military combat equipment.
The news agency had a slightly different video.
The current joint military exercises, known as Zapad, are among the largest Russia has conducted since the end of the Cold War.
They are being conducted jointly with close ally Belarus.
Officially, they involve 12,700 soldiers deployed in both countries, with most of the training taking place on Belarusian soil.
Western and Belarusian observers have put the number of Russian troops involved in the war games at between 70,000 and 120,000.
President Putin on Monday watched one of the Zapad drills at a training range near St Petersburg.