Major government and media websites of South and North Korea remained shut down Wednesday, after what appeared to be coordinated attacks during the anniversary of the Korean war.

Several South Korean websites were restored Wednesday, including the main page of the presidential Blue House and Chosun Ilbo newspaper, reports The Associated Press. Websites for the prime minister’s office, the science ministry and the country’s spy agency remain offline.

North Korea’s national airline Air Koryo, the ruling party’s Rodong Sinmun newspaper, the official Uriminzokkiri site and the country’s state-run Internet portal Naenara had been shut down Tuesday. All but Air Koryo were operational a few hours later.

South Korea’s National Intelligence Service officials said they were investigating the alleged hacking attacks and issued a nationwide cyber alert.

There was no immediate statement from Pyongyang either confirming or commenting on the attack, which coincided with the 63rd anniversary of the Korean War.

There have been allegations that the attacks were the work of the global “hacktivist” group Anonymous.

Anonymous denied any involvement but claimed they attacked North Korean websites on Tuesday, including the official Korean Central News Agency and Rodong Sinmun newspaper.

The two Koreas have traded accusations of recent cyber attacks. In March, 48,000 computers and servers in South Korea went down in an attack blamed on the North’s military-run spy agency.