Officials familiar with the intelligence said government analysts believe the missile could be a variant of North Korea's new Musudan intermediate-range missile, first disclosed publicly in October 2010.

Other intelligence indicates that the new ICBM may be under development at a huge missile testing facility on North Korea's western coast.

Prior to its mobile ICBM, North Korea's long-range missiles were the pad-launched Taepodong-1 prototype, and the Taepodong-2 (TD-2) dual-use ICBM and space launcher. The TD-2 was test-launched in April 2009.

Mobile missiles are difficult for tracking radar to locate, making them easier to hide. They also can be set up and launched much more quickly than missiles fired from silos or launchpads.