The long-range rocket launched by North Korea last month was largely made using domestic technology, South Korea says.
December's launch - condemned by its neighbours as a banned missile test - successfully put North Korea's first satellite in space.
International sanctions prevent North Korea importing advanced technology.
Correspondents say the discovery it was able to produce the "vast majority" of the parts without foreign help will be a cause for concern.
North Korea says the rocket carried a communications satellite into space, but the US and North Korea's neighbours have long believed such operations represent attempts by Pyongyang to develop long-range missiles.
South Korean military and space experts salvaged 10 pieces of the rocket which it shed into the sea as it took off, including its first-stage engine, and its fuel and oxidiser tanks.
In its report, the South Korean Defence Ministry said: "North Korea is believed to have made a majority of components itself, although it used commercially available products imported from overseas."
The BBC's Lucy Williamson in the South Korean capital, Seoul, says this will be unwelcome news to many countries as they consider what the military applications of such technical ability could be.
Such rockets are technically similar to intercontinental ballistic missiles, which could reach the United States, she adds.
North Korea has conducted two long-range rocket launches since Kim Jong-un came to power in December 2011. The launch in April failed, but December's attempt was an apparent success.
The US, Japan and South Korea are seeking a response in the UN Security Council, which banned North Korea from missile tests after nuclear tests in 2006 and 2009.
A South Korean diplomat said a draft resolution calling for tougher sanctions was being circulated at the UN, with a vote in the Security Council expected on Tuesday or Wednesday, South Korea's Yonhap news agency reports.