Mobile messaging apps Line and KakaoTalk remain out of service in China. The prolonged service outage is frustrating staff in both companies.
Naver's Line has been experiencing problems in China for more than a week, while users of KakaoTalk are able to use simple chat functions only.
A Naver spokesman said, "We're trying our best to resume service, but we still don't know what the source of the problem is and the Chinese government has not told us anything."
A Kakao spokesman said the problem involves blocked access to domains and is happening only in China. "We have to be careful in speculating about the possible causes," he added.
Yahoo's Flickr photo-sharing service as well as Microsoft's OneDrive cloud storage service is also blocked.
The Wall Street Journal and Bloomberg say the access problems are probably related to massive pro-democracy protests in Hong Kong on July 1. Around 500,000 people took part in the protest, which was timed to mark the 17th anniversary of the handover of Hong Kong to China from the U.K.
People posted photos and messages about the demonstration on social media or sent them via mobile messaging services so they spread to Taiwan and mainland China.
The WSJ said Beijing also blocked access to foreign websites and smartphone apps during sensitive events like the 25th anniversary of the Tiananmen Square massacre. The outages may also be related to the fact that foreign social media are tougher to censor than Chinese ones.
Other experts believe China may be out to protect its own interests. At present, China blocks access to some Google services as well as Facebook and Twitter in a move dubbed the Great Firewall.
One IT insider here said, "There are strong suspicions that the Chinese government is blocking access, but nobody dares mention that. It's a strong blow to the apps' competitiveness if service is unreliable."