The South Korean and U.S. militaries have postponed their annual joint drills out of concerns over a virus outbreak.

Thursday’s announcement came after South Korea reported 21 cases of a new coronavirus in its military and the U.S. military reported one case among its 28,500 troops in South Korea.

The announcement was jointly made by South Korea’s Joint Chiefs of Staff and the U.S. military in South Korea.

Kim Jun-rak, a spokesman at the South Korean Joint Chiefs of Staff, said the allies will put off their drills in the first half of this year until further notice.

Kim said JCS chief Park Han-ki first proposed putting off the drills out of concerns of safety of South Korean and U.S. troops.

He said Robert Abrams, the chief of the U.S. military in South Korea, accepted Park’s proposal after agreeing on the seriousness of the virus outbreak.

South Korea said Thursday it had more than 1,500 cases of the coronavirus that causes the COVID-19 illness, the second-most behind China.

The cases among military personnel have put thousands of South Korean troops in quarantine. The U.S. has closed facilities on several bases to stop the spread of the virus.