South Korea says it has raised its surveillance of North Korea after Pyongyang moved one or more intermediate-range missiles in readiness for a possible launch.
Admiral Samuel Locklear, the commander of American forces in the Pacific region, says the US military believes North Korea had moved an unspecified number of Musudan missiles to its east coast.
The North has been threatening the United States and South Korea on an almost a daily basis in recent weeks, although the threats appear to be aimed partly at boosting internal support for its young leader Kim Jong-un, Reuters reports.
Pyongyang has turned up its shrill rhetoric in recent weeks after the United Nations Security Council imposed sanctions for the state's third nuclear weapons test in February this year.
Meanwhile, UN secretary-general Ban Ki-moon has warned the crisis on the Korean peninsula may become uncontrollable. He is again urging North Korea to tone down "its provocative rhetoric" and to keep open a joint North-South Korean industrial complex.
Pyongyang has been making bellicose threats against South Korea, Japan and US bases, and now North Korean employees have not reported for work at the complex, suspending one of the few points of co-operation between the Koreas.
North Korea has asked foreign companies, organisations and tourists in South Korea to consider leaving for their own safety.
Following the threats issued by Pyongyang, Japan has deployed missile interceptors to the centre of Tokyo as a precautionary measure.