Seventeen American service members and six others affiliated with the US Forces Korea (USFK) tested positive for the new coronavirus upon arrival in South Korea from the United States, the US military said Monday.

The latest cases among the USFK-affiliated population rose to 509, most of whom have tested positive upon arrival in South Korea from the US.

Of the newly reported cases, eight service members and one dependent arrived at Osan Air Base in the city of Pyeongtaek, some 70 kilometers south of Seoul, on US government chartered flights from the US between Dec. 16 and 29, according to USFK.

Nine service members, three dependents, one contractor and one retired service member arrived on international commercial flights at Incheon International Airport, west of Seoul, between Dec. 16 and 31, it added.

Nine of them tested positive on their first mandatory COVID-19 test prior to entering quarantine. Two of them were confirmed to have been infected in a subsequent test while in quarantine, and the remainder tested positive in their mandatory test required to exit quarantine, the US military said.

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.

"Thorough cleaning of all transportation assets and quarantine rooms has been completed," USFK said in a statement. "None of the new arrivals have interacted with anyone residing within USFK installations or the local community."

Last week, USFK said it will maintain its alert level of the Health Protection Condition (HPCON) Charlie, or HPCON C, until Jan. 12, for all areas of South Korea.

Under the third-highest level in its five-tier system, only "mission essential" individuals report for duty, with all others teleworking. Non-necessary off-post activities are also restricted, such as visiting gyms outside the barracks, clubs, shopping centers and theaters. (Yonhap)