The long-range rocket fired by North Korea could have reached the US and eastern Europe, according to the South Korean defence ministry.
The launch was effectively the test of a ballistic missile capable of flying more than 10,000km (6,200) miles with a half-tonne payload, according to their scientists' analysis of the rocket's wreckage.
Its range covers the whole of Asia, eastern Europe, western Africa, Alaska and a large part of the US west coast including San Fransisco.
The estimates have been based on analysis of a container recovered from the rocket's first-stage splashdown site.
"Based on our analysis and simulation, the missile is capable of flying more than 10,000km with a warhead of 500-600 kilograms," a defence ministry official said.
However, without any debris from the second and third stages to analyse, the official said it could not be determined if the rocket had re-entry capability, which is a key element of inter-continental ballistic missile (ICBM) technology.
North Korea maintains the launch of the Unha-3 was not a missile test but simply designed to put the country's first satellite in space.
However, most of the rest of the world saw the launch as a disguised ballistic missile test in contravention of the UN resolutions imposed after Pyongyang conducted nuclear tests in 2006 and 2009.
South Korea is now analysing further wreckage from the rocket, including a fuel tank, a combustion chamber and an engine connection rod.
"As additional pieces have been salvaged, we will be able to look deeper into the function and structure of North Korea's long-range rocket," said the defence ministry official.
The read-out from the South Koreans comes after North Korea's leader Kim Jong-un made his first explicit call for the advancement of his country's long-range rocket programme.
On Friday, he gathered his rocket scientists together for a banquet in Pyongyang to urge them to build more powerful rockets.