Russia's top state investigative body says it is looking into a Russian media report alleging that sabotage experts from the United Kingdom's SAS special forces have been deployed to western Ukraine.
The Special Air Service is an elite military force trained to conduct special operations, surveillance and counter-terrorism.
Russia's RIA Novosti news agency on Saturday quoted a Russian security source as saying about 20 SAS members had been sent to the Lviv region.
In a statement, the Investigative Committee said it would follow up the report that they had been sent in "to assist the Ukrainian special services in organising sabotage on the territory of Ukraine".
A spokesperson for the British Ministry of Defence said: "We do not comment on Special Forces."
The UK said it sent military trainers to Ukraine earlier this year to instruct local forces in using anti-tank weapons but the UK government said on February 17 - a week before Russia's invasion - that it had pulled out all troops except those needed to protect its ambassador.
It was not clear what steps the Investigative Committee planned to take in response to any SAS involvement in Ukraine.
But the fact of the investigation into the possible presence of forces from a NATO country is significant, given that Russia has issued warnings to other countries not to get in the way of its "special military operation" in Ukraine.