Several people have been killed in explosions at two Russian military airfields, according to reports.
A fuel tanker exploded killing three and injuring six in an airfield near the city Ryazan, south-east of Moscow, Russian state media is reporting.
Another two people are reported to have been hurt in an explosion at an airfield in the Saratov region.
It is not known what caused the blasts. Both areas are hundreds of kilometres from the Ukrainian border.
Long-range Russian strategic bombers are believed to be based at the Engels airbase in the Saratov region.
The Saratov regional governor said security forces were checking what he called "reports of an incident at military installations".
These reports - of two explosions at two different military sites - will fuel speculation that Ukraine may be behind them, the BBC's Russian editor Steven Rosenberg says.
Ukraine has not officially commented.
However, presidential adviser Mykhaylo Podolyak appeared to refer to the reported incidents in a tweet, writing: "If something is launched into [an]other countries' airspace, sooner or later unknown flying objects will return to departure point."
Satellite images taken from Sunday appeared to show a large fleet of Russian heavy bombers at the Engels airbase.
They show what appeared to be type Tu-160 and Tu-95 strategic bombers, which are capable of firing cruise missiles - one of the weapons used in strikes on Ukraine.
Other satellite images released last week appeared to show increased military aircraft activity at the same airbase.
Russian President Vladimir Putin has been informed of the two reported incidents, Kremlin spokesman Dmitry Peskov said.
Asked to comment on the two explosions, Mr Peskov told his media briefing on Monday that he did not have information about the incidents and had only seen media reports about them.
Meanwhile, Ukraine has said that Russia has launched a new wave of missile strikes on the country, with air alerts declared in Kyiv and in most regions.
