The North Korean nuclear dispute has been raging for 22 months

US Under Secretary of State John Bolton told the BBC that Seoul had to abide by the Nuclear Non-Proliferation Treaty.

North Korea said the South's admission threatened a new nuclear arms race on the Korean peninsula.

Intense diplomatic efforts are being made in North Korea on Friday, with the visits of Chinese and UK officials.

A senior Chinese leader, Li Changchun, hopes to persuade North Korea back to the table for talks to resolve the stand-off over its nuclear programme.

China is due to host multilateral talks this month, but no date has yet been set, after North Korea pulled out.

A British minister, Bill Rammell, also visits Pyongyang on Friday. He has said he hopes to persuade the North Koreans to join the talks.

The BBC's Charles Scanlon says attempts to get North Korea to give up its nuclear ambitions will be made harder by the South's disclosures.

Admission

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.

It has become difficult to prevent expansion of a nuclear arms race because of South Korea's test Han Song-ryol North Korean envoy to UN Seoul battles disclosure fallout

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 International Atomic Energy Agency (IAEA) monitors had found evidence of it some years later.

He said the experiment had conformed to Seoul's commitments with the IAEA aimed at preventing the use of nuclear material for military purposes.

Last week, Seoul said 0.2 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.

'Dangerous'

The IAEA has been conducting an investigation and is expected to give a preliminary report in the coming days.

But 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.