There are fears that North Korea could be preparing to test-fire a long-range missile that may be able to reach America’s west coast.
It comes after Pyongyang said it was gearing up to launch a satellite on one of its rockets.
Intelligence officials have reported brisk personnel and vehicle activity at the Hwadae launch site.
North Korea’s longest-range missile, the Taepodong-2, has the potential to reach Alaska.
But reports suggest the missile being prepared for launch may be an advanced version of the Taepodong-2 that could reach the US west coast.
North Korea has only once tested the Taepodong-2, in 2006, when it flew for a few seconds and then exploded.
Pyongyang said: “The preparations for launching experimental communications satellite Kwangmyongsong-2 by means of delivery rocket Unha-2 are now making brisk headway.”