Reclusive North Korea has often issued threats to attack the South and the US, but has rarely turned them into action. Such hostile rhetoric is widely seen as a way to push its domestic and international political agenda.
A military spokesman for the North said the US faced "disastrous consequences" for moving a group of ships, including an aircraft carrier, into a South Korean port.
"The US will be wholly accountable for the unexpected horrible disaster to be met by its imperialist aggression forces' nuclear strike means," he said.
The order for the country's military to be on standby for combat was made by North Korean leader Kim Jong-un, the South's National Intelligence Service said.
South Korea's defence minister, Kim Kwan-jin, said later yesterday that there was no indication of unusual activity by the North's military.
Meanwhile, Washington brushed off the North's warning.
"We've seen this type of rhetoric from North Korea before," State Department spokeswoman Jen Psaki said. "Such comments from North Korea will do nothing to end [its] isolation or reduce the costs [it] pays for defying the international community."
The North has defied international warnings not to build nuclear and long-range missiles and is believed to have enough fissile material to build up to 10 nuclear bombs.
It also appears to have restarted its ageing nuclear reactor and conducted an engine test as part of its long-range missile programme.