Russian news outlet Kommersant said two Russian fighter jets and two military helicopters had been shot down on Saturday close to the Ukrainian border, in what would be a spectacular coup for Kyiv if confirmed.
Kommersant said on its website that the Su-34 fighter-bomber, Su-35 fighter and two Mi-8 helicopters had made up a raiding party, and had been "shot down almost simultaneously" in an ambush in the Bryansk region, adjoining northeast Ukraine.
"According to preliminary data ... the fighters were supposed to attack on targets in the Chernihiv region of Ukraine, and the helicopters were there to back them up - among other things to pick up the 'Su' crews if they were shot down."
The Russian state news agency TASS said a Russian Su-34 warplane had crashed in that region but did not specify a cause.
TASS also cited an emergency services official as saying an engine fire in a helicopter had caused it to crash near Klintsy, which is about 40 km (25 miles) from the border.
It made no mention of the Su-35, or of a second helicopter.
What caused the downings?
A video posted on the Russian pro-war Telegram channel Voyenniy Osvedomitel showed a helicopter high in the sky suffering an explosion, being thrown off course and then plunging towards the ground in flames.
Comments accompanying the video, which Reuters could not immediately verify, said it showed a Mi-8 being shot down by a missile. Other images posted by the channel showed wreckage in an agricultural field.
Kommersant provided no evidence for its report that four aircraft had been downed, but the same assertion was also made by several heavily followed pro-war military bloggers.
Later on Saturday, Bryansk regional governor Alexander Bogomaz said a Ukrainian drone had hit a food production plant in Starodub, close to the Ukraine border, damaging the roof. Bogomaz, writing on Telegram, said there were no casualties.
The incident occurred two days after Bogomaz reported a drone strike on an oil storage depot in the region.
The Moscow Defence Ministry did not immediately respond to a request for comment on the Kommersant report.
Voyenniy Osvedomitel said it appeared that "most likely, the enemy staged an ambush with air defenses previously transferred to a border zone close enough to hit our group."
It said the downed helicopters appeared to be Mi-8MTPR-1 electronic warfare craft.
Kommersant said all four crews had been killed.
There was no official response from Ukraine, which usually declines to comment on reports of attacks inside Russia.
However, in a Twitter post, Mykhailo Podolyak, a senior adviser to Ukrainian President Volodymyr Zelenskiy, called the incident "Justice ... and instant karma."
Earlier this month, explosions derailed freight trains on two consecutive days in the Bryansk region.