Dozens of computer networks in Ukraine have been infected by the aggressive “Snake” virus, according to experts.
The cyber weapon has been increasingly used since the start of this year, even before protests that led to the ouster of President Viktor Yanukovich, British-based BAE Systems said in a report published on Friday.
The complex composition of Snake bears similarities with Stuxnet, the malware that disrupted Iran's nuclear facilities in 2010.
Snake gives remote attackers "full remote access to the compromised system", the AFP news agency quoted BAE as saying.
Because it can stay inactive for a number of days, it is extremely hard to detect.
Although its origins are unclear, its developers appear to operate it in the same time zone as Moscow and some Russian text is embedded into the code, BAE said.
BAE has identified 14 cases of Snake in Ukraine since the start of 2014, compared to eight cases in the whole of 2013. In all there have been 32 reported cases in Ukraine since 2010, out of 56 worldwide.
Nigel Inkster, who until 2006 was the head of operations and intelligence at Britain's MI6 foreign intelligence agency, said Russia was most likely behind the cyber-attacks on Ukraine.
"Until recently the Russians have kept a low profile, but there's no doubt in my mind that they can do the full scope of cyber attacks, from denial of service to the very, very sophisticated."