The portion of virus cases with unidentified transmission routes reached a fresh record high of 26.4 percent of the total new infections identified over the past two weeks, health authorities said Thursday.

Of the 2,013 new cases reported over the past 14 days, 532 cases were grouped as infections with unknown transmission routes, according to the Korea Disease Control and Prevention Agency (KDCA).

It marked a fresh daily record high since health authorities began compiling related data in April.

The number of untraceable cases has topped 20 percent recently since mid-August, as cluster infections spiked in the densely populated Seoul area.

A rise in untraceable cases has put a strain on the virus fight, as it indicates that COVID-19 is spreading at a faster-than-expected pace in local communities and people can contract the virus unwittingly.

"The situation means that asymptomatic patients or people with mild symptoms can spread the virus additionally," KDCA Director Jeong Eun-kyeong said in a press briefing.

"It also indicates that health authorities face difficulties in carrying out contact tracing thoroughly," she added.

South Korea reported an uptick in daily new cases of COVID-19 on Thursday due to sporadic cluster infections and untraceable cases.

The country added 153 new coronavirus cases, including 145 local infections, raising the total caseload to 22,657, according to the KDCA. (Yonhap)