The former chief US negotiator in talks to rid North Korea of its nuclear weapons said Wednesday he was less optimistic than ever before about the prospects for a nuclear-free Korean peninsula.

Speaking during a visit to South Korea, Christopher Hill said the chances of persuading North Korea to give up its atomic weapons appeared bleak, according to Yonhap news agency.

"I think it's very clear at this point that it is a more difficult proposition than ever before," Yonhap quoted Hill as saying. "They have continued to work on their systems for delivering nuclear weapons."

Hill served as chief US delegate in the six-nation talks from 2005 to 2009 and helped negotiate deals that committed Pyongyang to dismantle its nuclear programme in exchange for various economic and political benefits.

The long-stalled forum, aimed at curbing Pyongyang's nuclear ambitions in exchange for aid, is chaired by the North's close ally China and also involves the two Koreas, the United States, Japan and Russia.

Pyongyang left the forum in April 2009 and conducted a second nuclear test a month later.

Hill added that China had a key role to play in persuading Pyongyang to give up its arsenal.

"China knows that Asia will not end up with a nuclear China and a nuclear North Korea -- that is, two nuclear nations. It will end up with many more nuclear weapons," he said.

"China, once they understand this clearly and once they develop the consensus to move forward, I think, can put pressure on North Korea."