A U.S. soldier stands guard in front of their Air F-16 fighter jet at Osan Air Base in Pyeongtaek.

U.S. forces in South Korea were put on their highest level of alert on Monday in case of any provocation from North Korea, following North Korea’s nuclear test last week.

Curtis Scaparrotti, Commander, U.N. Command/Combined Forces Command/United States Forces Korea (USFK), made the order during a visit to the Osan Air Base, operated jointly by the United States and South Korea, a USFK official said.

The U.S. and its ally South Korea are in talks towards sending further strategic U.S. assets to the Korean peninsula, a day after a U.S. B-52 bomber flew over South Korea in response to North Korea’s nuclear test last week.

“The United States and South Korea are continuously and closely having discussions on additional deployment of strategic assets,” Kim Min-seok, spokesman at the South Korean defense ministry said on Monday, declining to give specifics.

South Korean media said strategic assets Washington may utilize in Korea included B-2 bombers, nuclear-powered submarines and F-22 stealth fighter jets.

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