A delegation from the United Nations (search) nuclear watchdog arrived in South Korea on Sunday for a follow-up probe into the country's secret nuclear experiments.

The visit follows South Korea's recent admission that its scientists once dabbled in extracting plutonium and enriching uranium, both of which can be used to make nuclear arms.

South Korea says the experiments were purely research but has acknowledged it should have informed the International Atomic Energy Agency (search).

The five-member team declined to give details of its investigation and left soon after arriving for the Korea Atomic Energy Research Institute (search) in Daejeon, 125 miles south of Seoul, according to South Korea's Yonhap news agency. It was the second visit this month by a delegation from the U.N. agency.

The revelations of a South Korean nuclear history have threatened to disrupt already troubled efforts to hold another round of talks aimed at persuading North Korea to end its nuclear weapons program.

Pyongyang said Saturday that the United States was ignoring the nuclear activities of its allies while trying to pressure the communist North to give up its nuclear capability.

"South Korea's clandestine nuclear experiments go to prove that the U.S. double standards are a fundamental factor of the nuclear proliferation," said KCNA, the North's official news agency.

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