North Korea appeared to have lost access to the Internet Tuesday amid a cyber stand-off with the United States over the hacking of Sony pictures.

Major Internet sites based in the communist nation have been completely down since around 1 a.m. They include several sites run each by the Korean Central News Agency and the Rodong Sinmun, Pyongyang's two key mouthpieces.

There have been no problems accessing pro-Pyongyang web pages whose severs are located abroad, such as Uriminzokkiri and the Choson Sinbo.

While the source of Pyongyang's Internet blackout has not yet been confirmed, some observers here have raised the possibility of involvement by the U.S. government.

The U.S. authorities believe that North Korea was responsible for the recent hacking of Sony Pictures Entertainment, the producer of a comedy film about a plot to assassinate the country's leader Kim Jong-un.

President Barack Obama vowed a "proportional" response to what he called "cyber vandalism." (Yonhap)