The United States has placed the 28,500 troops it has stationed in South Korea on the highest level of readiness following North Korea's test of an atomic bomb, Yonhap news agency reported Monday.
Commander of the Republic of Korea-United States Combined Forces Curtis Scaparrotti issued the order during an inspection of the troops, the South Korean agency reported.
"I want you to maintain the highest-level readiness position with a long-term view because planned joint military exercises are ahead," Scaparrotti said.
South Korea and the US are scheduled to conduct annual joint training exercises at the end of February, Yonhap reported.
North Korea regularly accuses the US of using these exercises to prepare an attack on Pyongyang, a claim fiercely denied by Seoul and Washington.
During the troop inspection with Scaparrotti, South Korean General Lee Sun Jin cautioned that Pyongyang could feel itself pushed into provocative actions.
The US military sent a B-52 long-range bomber over the Korean peninsula Sunday in response to the nuclear test announced by North Korea on Wednesday. The plane then returned to its base on the Pacific island of Guam.
North Korea initially claimed that it had tested a hydrogen bomb, prompting worldwide outrage. Expert seismological analysis of the strength of the blast suggested later that it was in fact a conventional atomic bomb and not a much more powerful hydrogen bomb.
On Monday, North Korean leader Kim Jong Un encouraged researchers on the country's atomic bomb programme to work for "greater successes" on the nuclear weapons project.
Kim praised the scientists, technicians and workers for contributing to what North Korea maintains was its first successful test of a hydrogen bomb, state media reported on Monday.
The staff greeted Kim enthusiastically and the leader posed with them for a photo, official news agency KCNA said.
Kim said the test was an act of "self-defence."
North Korea carried out three atomic bomb tests between 2006 and 2013, leading to stiff sanctions imposed by the United Nations Security Council.