North Korea has accused the US of carrying out cyber attacks against its internet servers, blocking access to some websites.
Tensions are high between the North Korea and neighbouring South Korea and its US ally, after UN sanctions were tightened against North Korea following its latest nuclear test.
Foreigners in the capital Pyongyang reported no internet access on Wednesday and Thursday, but networks were operating normally by Friday, according to Fox News in the US.
South Korea's MBC television said the North's state media services were among those affected by the cyber attack, according to the Guardian.
These sites included those of the KCNA news agency and the official Rodong Sinmun newspaper, which were said to be experiencing disruptions.
KCNA said it was no secret the US and the South Korean “puppet regime” are bolstering cyber forces to intensify “subversive activities and sabotages” against North Korea.
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The news agency said intensive and persistent virus attacks were being made every day on internet servers operated by North Korea.
South Korea has denied the allegation, while the US military has declined to comment.
South Korea and the US are currently conducting joint military exercises on the Korean peninsula. North Korea has said it believes the alleged cyber attacks may be linked to those exercises, according to BBC reports.
Military experts believe cyber warfare is a major threat from North Korea alongside its conventional forces and weapons of mass destruction, posing a risk to South Korean critical infrastructure and communication networks.
According defectors from North Korea, the government is recruiting thousands of computer engineers to its cyber warfare unit.
Cyber attacks on critical national infrastructure is a top concern in the US, where president Barack Obama his issued an executive order requiring federal agencies to share cyber threat information with private companies.
The order also requires the creation of a cyber security framework aimed at reducing risks to companies providing critical infrastructure.
The concern is that once attackers gain access to key networks, they could cause physical damage to the infrastructure that the computers control before any intrusion is detected.