There are currently some 28,500 US troops stationed in the southern half of the Korean peninsula in a defence posture should the north cross the Demilitarized Zone. On Monday, the entire force was placed on the highest level of readiness.
"I want you to maintain the highest-level readiness position with a long-term view because planned joint military exercises are ahead," said Curtis Scaparrotti, Commander of the Republic of Korea-United States Combined Forces, according to Yonhap news agency.
The announcement comes after Pyongyang’s claims last week of having carried out a nuclear test, the fourth such test in the country’s history, and the first claim of a more powerful hydrogen-based weapon. Pyongyang insists that it is in possession of a hydrogen bomb, but experts maintain that the seismic tremors detected are too small to be the result of a hydrogen device detonation.
Despite its doubts, the US launched a show of strength against Pyongyang. On Sunday, the Pentagon flew a B-52 long-range bomber over the peninsula, escorted by a pair of South Korean F-15 fighters.
Pyongyang condemned that action as "pushing the situation to the brink of war."
As tensions increase, North Korea claimed to have arrested a naturalized American citizen on charges of spying for Seoul. Allowed to conduct an interview with CNN, 62-year-old Kim Dong-chul claims he was "tasked with taking photos of military secrets and 'scandalous' scenes" in North Korea.
The US State Department has neither confirmed nor denied Kim’s citizenship.
Next month, the United States and South Korea will conduct annual joint training exercises. North Korean leader Kim Jong Un has repeatedly cited the regional military drills by the allies as a security threat.
On Monday, the supreme leader of North Korea congratulated the scientists and technicians behind last week’s nuclear test, and encouraged researchers to work toward "greater successes," according to North Korean state media KCNA.
Kim Jong Un has maintained that the nuclear test was an act of "self-defense."
