South Korean officials have said North Korea is likely to have the missile technology to fire a warhead more than 10,000km.
The new technology, which was successfully tested earlier this month, would put the west coast of the US within range.
North Korea said the 12 December launch put a weather satellite in orbit.
However, critics say it was aimed at developing the kind of technology needed to mount a nuclear warhead on a long-range missile.
North Korea is banned from testing missile or nuclear technology under UN sanctions imposed after its 2006 and 2009 nuclear weapons tests and the UN Security Council condemned the launch.
South Korea retrieved and analysed parts of the first-stage rocket that dropped in the waters off its west coast.
"As a result of analysing the material of Unha-3 (North Korea's rocket), we judged North Korea had secured a range of more than 10,000km in case the warhead is 500-600kg," a South Korean Defence Ministry official told a news briefing.
North Korea's previous missile tests ended in failure.
It has spent decades and scarce resources to try to develop technology capable of striking targets as far away as the US and it is also working to build a nuclear arsenal.
However experts believe the North is still years away from mastering the technology needed to miniaturise a nuclear bomb to mount on a missile.
South Korean defence officials also said there was no confirmation whether the North had the re-entry technology needed for a payload to survive the heat and vibration without disintegrating.
Despite international condemnation, the launch this month was seen as a major boost domestically to the credibility of the North's young leader, Kim Jong-un, who took over when his father died last year.