SEOUL (AFP) - A state-owned Russian company has opened up a second Internet connection for North Korea which could strengthen Pyongyang's cyber capabilities and undermine US efforts to isolate the regime, security experts said.

The activation of the new line from TransTeleCom was first detected on Sunday (Oct 1) by analysts at Dyn Research, which monitors global Internet connectivity.

The new connection supplements the existing link provided by China Unicom, which has almost exclusively routed North Korean Internet traffic since 2010.

The additional line gives Pyongyang "significantly more resilience against attacks on their network infrastructure", said Bryce Boland, the chief technology officer in the Asia-Pacific for cybersecurity firm FireEye.

The Washington Post reported earlier that the US Cyber Command had carried out attacks against hackers in North Korea aimed at cutting off their access to the Internet.

The operation ended on Saturday, the report said.

North Korea has a 6,800-strong unit of trained cyberwarfare specialists, according to Seoul's defence ministry, and has been accused of launching high-profile cyber attacks, including the 2014 hacking of Sony Pictures.

But with only one Internet provider to rely on, the regime has often found itself vulnerable to external cyber attacks against its own network infrastructure.

North Korea suffered several Internet connection failures - some which lasted for hours - shortly after the Sony attack, which many suspected to be a US retaliation.

With the alternative route from Russia, "the possibility of disconnecting North Korea from the Internet just became much more difficult", Boland said.