A US Department of Defense (DoD) employee working in Seoul and two of his family members have tested positive for the new coronavirus, the US Forces Korea (USFK) said Thursday.

The employee, his spouse and their dependent were confirmed to have contracted the virus Wednesday following their contact with another DoD official who previously tested positive, according to the US military.

The civilian employee and his spouse work at US Army Garrison Yongsan in central Seoul, and they last visited the installation on Tuesday.

All three are now in isolation at a facility designated for COVID-19 patients at Camp Humphreys in Pyeongtaek, some 70 kilometers south of the capital.

"KDCA and USFK health professionals are actively conducting contact tracing to determine whether anyone else may have been exposed to these individuals, and to ensure all known on-post and off-post facilities visited are thoroughly identified and cleaned," USFK said in a release. KDCA stands for the Korea Disease Control and Prevention Agency.

Later in the day, the US military said 21 service members and four dependents have tested positive upon arrival in South Korea between Dec. 22 and Wednesday.

Of them, 15 service members and the four dependents arrived at Osan Air Base in Pyeongtaek on US government chartered flights while the others arrived here via commercial flights.

All USFK-affiliated individuals arriving in South Korea are required to undergo a virus test and quarantine for 14 days. Medical personnel administer a second test prior to their release.

The newly confirmed patients have all been transferred to isolation facilities.

"None of the new arrivals have interacted with anyone residing within USFK installations or the local community," USFK said.

The latest cases raised the total number of virus cases among USFK-related people to 538. (Yonhap)