Intense diplomatic efforts are under way to persuade North Korea to give up its nuclear weapons programme.
South Korea said its nuclear tests were on a small scale
Delegations of Chinese and British officials are holding talks in the North's capital, Pyongyang.
The BBC's Charles Scanlon in Seoul says they will be hoping South Korea's recent admission of nuclear experiments will not undermine their efforts.
The US has warned Seoul to expect no favours after it admitted experimenting with plutonium and uranium.
US Under Secretary of State John Bolton told the BBC that Seoul had to abide by the Nuclear Non-Proliferation Treaty.
North Korea - which itself has a well advanced atomic weapons programme - has said the South's admission threatens a new nuclear arms race on the Korean peninsula.
A senior Chinese leader, Li Changchun, is leading a delegation that aims to persuade Pyongyang to return to multilateral talks aimed at ending its nuclear programme.
China is due to host the next round of talks - also involving both Koreas, Japan, the US and Russia - this month, but no date has been set for the meeting after North Korea said it would not attend.
The North Korean nuclear dispute has been raging for 22 months
British Foreign Minister Bill Rammell, the first UK minister to visit North Korea, said the South's activities did not give the North an excuse to continue its programme.
"They [South Korea] are co-operating [with the IAEA] and that's different from a nation that has thrown out inspectors, admitted it has enriched uranium then denied it," he said.
Meanwhile, the chief US negotiator on North Korea, James Kelly, is in Tokyo to discuss the proposed talks with his diplomatic partners.
"We are very much interested in having six-party talks by the end of September... and we were talking today about how to do that," he told reporters.
However, South Korean Foreign Minister Ban Ki-moon has expressed pessimism about the prospects of the meeting going ahead.
"It is becoming difficult to be optimistic," he told South Korean radio.
On Thursday, Seoul admitted it had extracted a small amount of plutonium - a key ingredient in nuclear bombs - in secret research conducted in the early 1980s.
An official from South Korea's science and technology ministry, Kim Young-shik, said scientists had conducted an unauthorised experiment out of academic curiosity.
He said the experiment had conformed to Seoul's commitments with the International Atomic Energy Agency aimed at preventing the use of nuclear material for military purposes.
Last week, Seoul admitted that a fifth of a gram of uranium was produced in 2000 by scientists who did not have government approval.
Officials insist the uranium experiment was conducted for South Korea's civilian nuclear power industry.
The IAEA has been conducting an investigation and is expected to give a preliminary report in the coming days.
Many questions are now being asked of South Korea's experiments, our correspondent says.
North Korea's UN envoy Han Song-ryol described the South's "nuclear experiment" as a "dangerous move".
"We view South Korea's uranium enrichment programme in the context of a nuclear arms race in north-east Asia," Mr Han told South's Yonhap news agency.
"It has become difficult to prevent expansion of a nuclear arms race because of South Korea's test," he said.