"We evaluate that North Korea can produce nuclear weapons using uranium," Kim Kwan-Jin was quoted by a news agency as telling members of parliament. (Agencies)
He did not give details but it marked a rare assessment by a top South Korean official of the North's nuclear capability.
North Korea has carried out three nuclear tests since the first in 2006, the last, and most powerful, in February this year.
The two previous tests used plutonium but it remains unknown whether the last test involved enriched uranium.
Satellite images suggest the North has restarted a plutonium-producing reactor at its main Yongbyon nuclear complex, and also doubled its uranium enrichment capacity at the same site.
Kim said Seoul was closely monitoring activities at Yongbyon. "We suspect the North is testing the reactor and we are keeping a close eye on it," Kim said.
With the nuclear programmes, Pyongyang aims to extract international concessions, secure the status of a "nuclear weapons state", hold its regime together and gain the upper hand over the South through nuclear blackmail, Kim said.
A new study by Washington-based nuclear proliferation expert Joshua Pollack and nuclear scientist Scott Kemp, of the Massachusetts Institute of Technology, suggests North Korea is capable of indigenously producing the key components of the gas centrifuges needed to enrich uranium.
Uranium enrichment carries a far smaller footprint than plutonium. It can be carried out using centrifuge cascades in relatively small buildings that give off no heat and are less visible from satellites.
The North's current stockpile of nuclear material is variously estimated as being enough for six to 10 bombs. It is unclear whether it has managed to create a nuclear warhead for a missile.
"We evaluate that North Korea can produce nuclear weapons using uranium," Kim Kwan-Jin was quoted by a news agency as telling members of parliament.