A previously unknown hacking group variously dubbed "Strider" or "ProjectSauron" has carried out cyber-espionage attacks against select targets in Russia,China, Iran, Sweden, Belgium and Rwanda, security researchers said on Monday.

The group, which has been active since at least 2011 and could have links to a national intelligence agency, uses Remsec, an advanced piece of hidden malware, Symantec researchers said in a blog post.

Remsec spyware lives within an organisation's network rather than being installed on individual computers, giving attackers complete control over infected machines, researchers said.

It enables keystroke logging and the theft of files and other data. Its code also contains references to Sauron, the all-seeing title character in The Lord of the Rings, Symantec said.

Strider is the nickname of the fantasy trilogy´s widely travelled main character Aragorn.

Separately, Moscow-based Kaspersky Lab has labelled the same group using the Remsec spyware as "ProjectSauron".

The newly discovered group's targets include four organizations and individuals located in Russia, an airline in China, an organization in Sweden and an embassy in Belgium, Symantec said.

Kaspersky said it had found 30 organisations hit so far in Russia, Iran and Rwanda, and possibly additional victims in Italian-speaking countries.

Remsec targets included government agencies, scientific research centres, military entities, telecoms providers and financial institutions, Kaspersky said.

"Based on the espionage capabilities of its malware and the nature of its known targets, it is possible that the group is a nation state-level attacker," Symantec said, but it did not speculate about which government might be behind the software.

Despite headlines that suggest an endless stream of new types of cyber-spying attacks, Orla Fox, Symantec´s director of security response said the discovery of a new class of spyware like Remsec is a relatively rare event, with the industry uncovering no more than one or two such campaigns per year.

Remsec shares certain unusual coding similarities with another older piece of nation state-grade malware known as Flamer, or Flame, according to Symantec.

Kaspersky agreed that the same group it calls ProjectSauron appears to have adopted the tools and techniques of other better-known spyware, including Flame, but said it does not believe that ProjectSauron and Flame are directly connected.

Flamer malware has been linked to Stuxnet, a military-grade computer virus alleged by security experts to have been used by the United States and Israel to attack Iran's nuclear programme late in the last decade.