At least one Russian warplane and a military helicopter have crashed in the Bryansk region bordering Ukraine, Russian emergency services were quoted by TASS news agency as saying, with several reports claiming the craft had been shot down.
Later, Russian newspaper Kommersant reported that a Russian SU-35 jet and a second Mi-8 helicopter also crashed on May 13, but that report could not immediately be confirmed.
Kommersant said an SU-34, SU-35, and two Mi-8 helicopters were part of an air group and that they had been "shot down almost simultaneously" in the Bryansk region.
"According to preliminary data, they were all part of the same air group -- the fighters were supposed to deliver a missile and bomb attack on targets in the Chernihiv region of Ukraine, and the helicopters were to secure them, in order to pick up the SU crews if they were shot down by enemy fire," the report said.
"All four aircraft failed to return to the airfield. Their pilots are dead," it added.
Kommersant did not provide evidence for its claim that four aircraft had been shot down, but several Russian military bloggers also reported the same.
The Telegram channel Baza said a total of nine personnel -- members of the crew of two Mi-8 helicopters and SU-34 and SU-35 aircraft -- had been killed.
Kyiv did not comment on the reports. Ukrainian authorities normally do not comment directly on any incidents occurring within Russia’s borders.
If confirmed that the aircraft were shot down, it would represent a major coup for the Ukrainian military and an embarrassment for the Kremlin.
Russia's TASS news agency did not mention the reports of the downings of the second jet and the second helicopter.
It said an SU-34 fighter jet crashed "not far from the Ukrainian border," without giving additional details.
Earlier, an Mi-8 helicopter crashed in Klintsy, some 45 kilometers north of the Ukrainian border, killing two people on board, the agency said, adding that an engine fire was the cause of the crash.
A Russian Mi-28 attack helicopter crashed on May 12 in Ukraine's Moscow-occupied Crimea, killing two.
Bryansk Governor Aleksandr Bogomaz confirmed the fall of one Mi-8 helicopter in Klintsy, as well as a report about an injured woman on the ground and damage to five houses.
"Today, a helicopter crashed in Klintsy. As a result of the incident, there is one victim among civilians -- a woman. Currently, she has been taken to the district hospital, she is receiving all the necessary medical care," he wrote on Telegram.