The BBC reports:
"Russian state media claims that "sabotage specialists" from the UK are helping the Ukrainian military in the western Lviv region.
According to the Ria Novosti news agency, at least two groups - or about 20 people in total - from the SAS have been deployed.
"These are no ordinary special forces," a Russian intelligence source is quoted as telling Novosti.
The source added that the SAS personnel are likely "co-ordinating the activities of sabotage groups" in Russian-controlled territory.
British media reported last week that SAS troops had been training local forces, according to Ukrainian commanders.
Founded during World War Two, the Special Air Service is known for covert missions, including hostage recovery, reconnaissance and counter-terrorism."