The four-member International Atomic Energy Agency (IAEA) team is on a week-long mission to visit two state nuclear centers and interview scientists as a follow-up inspection, officials said.

Yonhap news agency said late Sunday the inspection team reached by car Daejeon, 160 kilometers (100 miles) south of Seoul, to visit the Korea Atomic Energy Research Institute.

Yonhap said one more inspector is later to join the inspection team which should report back to the Vienna-based IAEA by November.

Its initial investigation began three weeks ago after Seoul's shock revelations that its scientists secretly extracted a tiny amount of plutonium in 1982 and enriched uranium in 2000.

South Korea says the lab experiments had been for scientific purposes irrelevant to nuclear weapons programs. It denies seeking or possessing nuclear arms.

But the case has already damaged multinational efforts to persuade Stalinist North Korea to give up its nuclear weapons programs.

The chances are slim for a resumption this month of six-nation talks on the issue as Pyongyang has hardened its position on Seoul's past nuclear activities.

Pyongyang's official Korean Central News Agency said Saturday that North Korea would never dismantle its nuclear deterrent, and that six-way talks could not be resumed unless Washington changed its policy towards Pyongyang.

"It is self-evident that the resumption of the talks can no longer be discussed unless the US drops its hostile policy based on double standards toward the DPRK (North Korea) and that the latter can never dismantle its nuclear deterrent force," it said.

A North Korean foreign ministry spokesman also said Friday Pyongyang "can never sit at the table to negotiate its nuclear weapon program unless the truth about the secret nuclear experiments in South Korea is fully probed".

China, host of the six-way talks, admitted Thursday it would be difficult to hold the talks by the end of September as planned.

But Seoul urged Pyongyang to return to the talks which bring together the two Koreas, China, the United States, Russia and Japan.

"The (South Korean) nuclear experiments have nothing to do with the North Korean nuclear issue and it must have no impact on the six-nation talks," South Korean foreign ministry spokesman Lee Kyu-Hyung said Friday.

South Korea, a nuclear Non-Proliferation Treaty signatory, on Saturday made a fresh pledge not to develop and own nuclear weapons.

A US State Department spokesman has said Washington sees the South's research as "laboratory experiments" and not as nuclear weapons activities.

The stand-off over North Korea's nuclear ambitions flared in October 2002 when Washington accused Pyongyang of operating a nuclear weapons program based on enriched uranium, violating a 1994 agreement.

Pyongyang has denied running the uranium-based program but has restarted its plutonium program.

Scientists at the state institute produced 150 kilograms (330 pounds) of uranium metal in 1982 in undeclared activities and a small amount of this was used in 2000 to produce the enriched uranium.

They also admitted to having extracted a miniscule amount of plutonium from 2.5 kilograms of fuel rods in secret research in 1982.

IAEA chief Mohamed ElBaradei last week expressed "serious concern" about the activities.

South Korea has the world's sixth-largest civilian nuclear industry, operating 19 nuclear power plants that produce 40 percent of the country's energy needs.