The South’s Yonhap news agency quoted an official, speaking under terms of anonymity, as saying the government was looking to see whether there had been "external hacking" of the North’s limited Internet service.
"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," an official told reporters.
North Korea’s internet was shut down for several hours, Yonhap reported, with the home pages of Korean Central News Agency and the Rodong Sinmun newspaper, two of Pyongyang's key media outlets, returning around 11.00 local time Tuesday (05.00 GMT).
U.S. President Barack Obama said over the weekend that the U.S. would “respond proportionately” to last month’s cyber attack on Sony Pictures, which the U.S. blamed on North Korea.
Washington and Pyongyang have declined to comment on the North's Internet outage.
U.S. State Department spokeswoman Marie Harf said: “We aren’t going to discuss… publicly operational details about the possible response options or comment on those kind of reports in any way except to say that as we implement our responses, some will be seen, some may not be seen.”
The North has denied responsibility for the Sony attack.
North Korea experienced progressively worsening Internet disruption from Saturday, the day after the U.S. officially accused Pyongyang of being behind the Sony attack. The hack is seen as retribution for the production and planned distribution of the movie “The Interview,” which features the fictionalized assassination of North Korean leader Kim Jong-un.
Arbor Networks, an Internet technology service, said it had observed denial-of-service attacks, which overwhelm websites with massive amounts of online traffic, on North Korea’s web infrastructure.
North Korea has limited connectivity and relatively unsophisticated Internet services, meaning it would be relatively simple to shut down web access. Few North Koreans have Internet access and those that do can usually only see heavily censored content.
Internet experts also said it was possible North Korea had experienced a software problem with its Internet routers, although the outage did not fit the pattern of router instability.
Meanwhile, South Korean President Park Geun-hye called on officials in her country to "fundamentally check preparations against cyber terrorism" on key facilities.
Park highlighted the example of the Sony hack at a cabinet meeting, Yonhap reported. Seoul has also accused North Korea of carrying out numerous cyber attacks, including on banks and media outlets last year.
On MondaySouth Korea's nuclear plant operator, Korea Hydro and Nuclear Power, reported that it had been hacked and data stolen but said there was no risk to nuclear installations.
Officials said North Korean involvement in the attack could not be ruled out and that the hacking carried some similarities to previous attacks in which the North had been implicated.
On Monday evening, the UN Security Council tabled an agenda item on the North's human rights record. Pyongyang suggesting it would carry out a banned nuclear test in retaliation.