Kakao Pay, the online payment unit of Korean tech giant Kakao, experienced another service disruption for some five hours on Sunday due to a delayed firewall replacement.

Last Monday, Kakao Pay initially announced that it would be replacing its firewall from 1 a.m. to 7 a.m. on Sunday. However, due to an unexpected delay, its key services such as processing payments, sending money and asset management became unavailable on Sunday morning.

Kakao Pay’s new announcement on Sunday morning said that it would complete the firewall replacement process by noon, and the service was normalized a little earlier at 11:37 a.m.

The payment firm's spokesperson declined to comment on the situation on Sunday, adding that an announcement about the restoration of the service had been made to clients on Sunday.

“Everything is back to normal, and we sincerely apologize for any inconveniences,” Kakao Pay’s announcement said.

Providing stable online services has been one of Kakao Pay's key issues after experiencing a string of service outage cases, which have harmed customers' trust.

In October of last year, Kakao suffered its worst server outage so far due to a fire at its data center. A broad range of its services, including the ability to make payments, were affected by this outage. Following this incident, the firm also experienced service outages in January and May this year.

The latest incident took place only three days after the tech company released the Kakao Stability Report, which outlines measures and action plans for service stability on Thursday.

In the report, Kakao emphasized technical measures to prepare for data center outages and increased investment to strengthen service stability.

"Kakao always prepares for all possible situations, and even if unexpected failures occur, we make sure to respond quickly so that users can maintain their normal daily life,” Kakao CEO Hong Eun-taek said when releasing the report.

Kakao mentioned that it has established an environment where it can immediately execute recovery measures in the event of a failure by multiplexing its cloud, operational tools and data centers.

The firm said it has also increased its investments in enhancing service stability. The amount that Kakao used for information security amounted to over 20.9 billion won ($15.72 million) this year, which is an approximately 48.8 percent on-year increase.