Several major North Korean websites are experiencing a fourth day of service disruption Friday, following major outages earlier this week. The disruption to the country's Internet service has prompted speculation that it may be the result of a U.S. response to the hacking of Sony Pictures.

The website of the state-run Korean Central News Agency was blocked for a time early Friday, and is now only intermittently accessible. In addition, the websites of several other government-run propaganda organs have experienced similar problems, according to South Korean news agency Yonhap

The continued instability follows two days when the country's entire access to the Internet was cut off, and it has suffered intermittent disruption to services since then.

U.S. President Barack Obama had pledged to “respond proportionately” to the hacking of Sony Pictures, which the U.S. government is blaming on North Korea, leading many to speculate that the U.S. may have been behind the outage.

The U.S., however, has denied any involvement. State Department spokeswoman Marie Harf said Tuesday that she “could not speak to” the situation, and suggested that questions should be directed to the North Koreans, according to The Telegraph

Internet security expert Matthew Prince, CEO of U.S.-based CloudFlare, told Reuters the fact that North Korea's Internet was back up "is pretty good evidence that the outage wasn't caused by a state-sponsored attack, otherwise it'd likely still be down for the count."

Access to the Internet is severely limited in North Korea, with only high-level officials, diplomats and NGOs allowed to access the Web. Some North Korean university students have access to the Internet, but their activity is monitored, while a small number of citizens have access to a national “Intranet” that features government-approved news.

As a result, the outages are unlikely to have a significant impact on the people of North Korea. Ofer Gayer, a security researcher at Incapsula Inc., told the Associated Press: “A large city block in London or New York would have more IP (Internet Protocol) addresses than North Korea.”

Despite the U.S. government publicly laying the blame for Sony hack at the feet of North Korea, some online security experts remain unconvinced, as the government's view is based on evidence that it has not made public. “Essentially, we are being left in a position where we are expected to just take agency promises at face value,” Marc Rogers, a security researcher at CloudFlare, the mobile security company, wrote in a post cited by the New York Times. “In the current climate, that is a big ask.”