The official said the missile was expected to be ready for test-firing by February 25 at the earliest.
The report said US spy satellites were tracking preparations and would be able to provide several days' warning of an imminent launch.
South Korea has said that any missile test would bring the North increased international isolation and sanctions, while the US has warned it would view a launch as provocative.
The Taepodong-2 missile is believed to be capable of flying as far as Alaska or the US west coast but has never successfully flown.
The only previous test in July 2006 saw the missile blow apart about 40 seconds after launch.
According to South Korean officials North Korea may also be preparing to launch short-range missiles near a disputed sea border off the peninsula's west coast.
Analysts say the missile tests are intended to put pressure on Lee Myung-bak, the South Korean president, to drop his tough stance against the North and abandon what Pyongyang says is his "confrontational policy".
It is also thought to be aimed at persuading Barack Obama, the US president, to make North Korea one of his top foreign policy priorities.
The issue of North Korea's weapons programmes is expected to be one of the key issues on the agenda of Hillary Clinton, the US Secretary of State, when she visits Asia next week in her first overseas trip since taking office.
North Korea, which tested a nuclear bomb in 2006, is not yet thought to have mastered the technology to miniaturise an atomic weapon to mount as a warhead on top of a missile.
It does though have several hundred conventional ballistic missiles within range of South Korea and Japan.