South Korea has raised its alert level to “vital threat” amid indications the North is preparing for a missile test.
At least one ballistic missile with an estimated 3,000km (2,000-mile) range is fuelled and ready for launch, US and South Korean sources say.
Pyongyang has been making bellicose threats against South Korea, Japan and US bases in the region.
The threats follow tough new UN sanctions imposed on North Korea last month after its third nuclear test.
Separately, an initial investigation by the South into a major cyber attack last month that affected a number of banks and broadcasters has said the North is to blame.