Thirteen American service members and two civilians affiliated with the US Forces Korea (USFK) have tested positive for the coronavirus upon arrival here from the United States recently, USFK said Monday.

Eleven service members, one civilian and one contractor arrived on international commercial flights at Incheon International Airport, west of Seoul, between Jan. 21 and Feb. 6, according to USFK.

The other two service members arrived at Osan Air Base in Pyeongtaek, around 70 kilometers south of Seoul, on US government-chartered flights from the US over the past two weeks, it added.

Ten of the total tested positive on their first mandatory COVID-19 test prior to entering quarantine, and the five others tested positive on their mandatory test required to exit quarantine.

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.

"All individuals have since been transferred to an isolation facility designated for confirmed COVID-19 cases at US Army Garrison Humphreys, Osan and Kunsan Air Bases," USFK said in a statement. "Thorough cleaning of all transportation assets and quarantine rooms has been completed."

Meanwhile, USFK said in a separate statement that four service members mistakenly tested positive for the coronavirus last week.

On Friday, the command reported nine more COVID-19 patients among its population, including six US service members stationed in South Korea. But four of them were "actually 'false positive'" as "a defective manufactured reagent used during the original PCR testing machine" was found to have caused erroneous readings.

USFK noted that multiple retests of the cases, in-depth research and discussion have led it to such a conclusion, and the four people have been in isolation in Camp Humphreys in Pyeongtaek.

With the latest developments, the total number of COVID-19 infections among the USFK members stood at 721.

On Monday, USFK lowered its antivirus alert level of the Health Protection Condition (HPCON) for Area IV that includes the southeastern and southwestern provinces of Gyeongsang and Jeolla, respectively, by one notch to the second-lowest "Bravo" from "Charlie," citing a "reduced threat of the virus" within the region.

All other USFK designated areas remain in HPCON Charlie until further notice, according to USFK.

Under the eased restrictions, seated dining at off-installation restaurants, domestic travel with overnight stays and shopping at indoor malls are allowed while following South Korea's social distancing rules.

But bars, clubs and other facilities that restrict entrance to adults still remain off limits to all USFK members across the country, USFK said.

On Monday, South Korea added 289 more COVID-19 cases, including 264 local infections, marking the lowest number since late November when another wave of the virus began. (Yonhap)