North Korea's major websites resumed their normal operations Tuesday after an hours-long outage that came amid a cyber stand-off with the United States over the hacking of Sony Pictures.

The home pages of Korean Central News Agency and the Rodong Sinmun, two of Pyongyang's key mouthpieces, were back online as of around 11 a.m.

But speculation continued to grow over the reason for the Internet blackout.

"We are closely monitoring to find out whether the reason for the web outage was an external hacking or internal system check to boost security," a South Korean government official told reporters.

Some observers here raised the possibility of a retaliatory attack by civilian hackers or the U.S. government.

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)