Amid heightened alert over new coronavirus infections in the metropolitan area, the number of new cases reported in South Korea's capital increased by 25 on Thursday, due mostly to a sharp rise in infections traced to a facility for the elderly.

All but one of the 25 cases were locally transmitted. Eleven were linked to a day care center for senior citizens in the northeastern ward of Dobong, which raised the number of related cases to 38, according to the Korea Centers for Disease Control and Prevention (KCDC).

Two of the cases were linked to a door-to-door health product seller whose target customers were the elderly. The company has emerged as one of the biggest infection clusters in the capital, with 180 related cases identified nationally.

Three cases were linked to a mass infection cluster at small churches and religious meetings in the metropolitan area, while one was traced to a table tennis club in western Seoul.

In addition to the cases confirmed by the KCDC, sporadic infections were reported across the city, including a lecturer at an English language institute in the southern Seoul ward of Seocho.

The persistent infections related to the day care center and the health product seller have put health authorities on alert, as most of the patients are the elderly, who are likely to suffer more from the infectious virus.

The country's average mortality rate of new coronavirus patients stand at 2.28, but the figures rise for elderly patients.

The mortality rate is 9.91 percent for those in their 70s and 25.69 percent for those in their 80s or older. 

South Korea in total added 59 cases, the most in three weeks, bringing the country's COVID-19 caseload to 12,257, according to the Korea Centers for Disease Control and Prevention. (Yonhap)