US forces in South Korea have been placed on their highest level of alert in case of any provocation from North Korea, after North Korea's nuclear test last week.
Curtis Scaparrotti, Commander, UN 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 US and South Korea have also been in talks about sending further strategic US assets to the Korean peninsula, a day after a US B-52 bomber flew over South Korea in response to the nuclear test.
"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 defence ministry said on Monday (local time), declining to give specifics.
South Korean media said strategic assets Washington may utilise in Korea included B-2 bombers, nuclear-powered submarines and F-22 stealth fighter jets.
Seoul also said on Monday that it would restrict access to the jointly run Kaesong industrial complex just north of the heavily militarised inter-Korean border to the "minimum necessary level" starting from Tuesday.
North Korea says it exploded a hydrogen bomb last Wednesday, although the United States and outside experts doubt that the North had achieved such a technological advance in its fourth nuclear test. The test angered China, the North's main ally, which was not given advance notice, and the United States.
In a show of force and support for its allies in the region, the United States on Sunday sent a nuclear-capable B-52 bomber based in Guam on a flight over South Korea.
Separately, South Korea and Japan used their shared military hotline for the first time in the aftermath of North Korea's nuclear test, Seoul's defence ministry said, a sign the North's provocation is pushing the two long-time rivals, which are Washington's main allies in the region, closer together.
South Korea has also resumed anti-North propaganda broadcasts using loudspeakers along the border, a tactic that the North considers insulting and resulted in an armed standoff that included an exchange of artillery fire the last time South Korea used the speakers in August.
South Korea's president Park Geun-hye plans to make a speech to the nation on Wednesday in which she is expected to express strong will to respond to North Korea's nuclear test, a presidential official said.