KT said the Internet service was interrupted in parts of southern Seoul, with areas including the Sinsa, Yangjae, Suseo and Seocho areas being affected.
The company said the latest disruption was caused by efforts to upgrade its fixed-line Internet modem.
KT further said it is working to fix the problem as quickly as possible, with the issue likely to be resolved within the day.
The company is currently taking steps to offer better all-around service to customers, and is poised to commence super-fast 5G network service early next month.
The disruptions come after a fire broke out at KT's communication building in November 2018 that paralyzed the company's mobile and Internet services, including card transactions, in large parts of the capital for days.
The blaze was extinguished 10 hours after it started. No one was injured in the incident, although the property damage was estimated at 8 billion won ($7 million). (Yonhap)