By Charles Scanlon
BBC News, Seoul
South Korea's chief nuclear negotiator says North Korea has tried to develop a second, secret nuclear programme based on the enrichment of uranium.
The nuclear disarmament deal was agreed at six-party talks
Chun Yung-woo said, however, that Pyongyang was not thought to be operating such a system at the moment.
The North has long denied American allegations that it was building a uranium programme in addition to its publicly acknowledged plutonium plant.
The dispute is seen as a major obstacle to implementing a nuclear agreement.
Under the deal, reached this month, the North has agreed to shut down its nuclear facilities in return for economic aid.
North Korea has agreed to produce a list of all its nuclear facilities as a first step towards disabling them in line with the agreement.
But it continues to deny the existence of an alleged uranium programme which led to the breakdown of its last nuclear deal with the United States.
Mr Chun said the North's attempts to acquire parts for such a programme were well known to countries that monitor the illicit trade.
But he said it was not clear how far the North had got, and no-one thought it was actively enriching uranium for nuclear weapons at the moment.
North Korea has agreed to shut down its nuclear reactor at Yongbyon, the source of the plutonium for its existing nuclear arsenal.
But analysts say the dispute over uranium could derail further progress.
The chief US negotiator, Christopher Hill, said in Washington it was a very serious problem that had to be addressed.
South Korea said the recent deal is just the first step in a very long process.
It said much will depend on improving the political climate and the deep mistrust between the United States and North Korea.