U.S. Forces Korea (USFK) raised its health protection alert on Saturday after posting its highest weekly tally of new coronavirus infections at 682.
The new cases bring the total infections to 3,027. U.S. forces has said nearly 90% of its soldiers, families and other affiliated people were vaccinated.
In neighboring Japan, Prime Minister Fumio Kishida said Friday the government would tighten curbs in three prefectures that host U.S. military bases to stem a COVID-19 surge that some officials have said the bases have helped fuel.
The U.S. military's "Bravo Plus" plan in South Korea will ban dining at restaurants outside the bases and visiting indoor malls, bars, clubs, gyms, amusement parks, theaters and massage parlors, U.S. forces said on its website on Friday.
Travel to Seoul is also prohibited, except for official duties or for people who live or work there, but activities including grocery shopping and religious services are allowed.
"Due to the continued presence of COVID-19 within USFK and South Korea, USFK has increased its health protection condition to 'Bravo Plus' peninsula-wide," it said.
South Korea had reported record-breaking daily infections of around 7,800 after it eased social distancing rules under a "living with COVID-19" plan in November, prompting authorities to reinstate tougher curbs the following month.
The Korea Disease Control and Prevention Agency reported 3,510 new cases for Friday, bringing the total to 661,015, with 5,986 deaths.
The defense ministry said 19 new infections were confirmed among South Korean military personnel as of Friday, for a total of 3,403.