The long-range rocket launched by North Korea last month was largely made using domestic technology, South Korea said, according to BBC News.
December's launch – condemned by its neighbors 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 U.S. and North Korea's neighbors 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."
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 U.S., 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, Jan 22 or Wednesday, South Korea's Yonhap news agency reported.
