Top U.S. nuclear negotiator Christopher Hill said on Wednesday North Korea could move ahead to "disable" its nuclear arms programmes by the end of this year, adding it was important to remain focused.

Hill, assistant secretary of state for East Asia and Pacific affairs, made the remarks ahead of a new round of six-party talks involving the two Koreas, China, Japan, Russia and the United States, which are scheduled to begin in Beijing on Thursday.

Under a February 13 agreement, North Korea must disable its nuclear facilities and give a complete declaration of all its nuclear programmes. In return, it is to receive 950,000 tonnes of heavy fuel or the equivalent.

"We're looking forward to having disablement and declaration by the end of the year. It's a very ambitious process, but I think it's doable," Hill told reporters on arrival in Beijing.

"I just believe we've really prepared very well for this meeting and whenever you prepare well for a meeting I think that's the best way to go into a meeting," he added.

But Hill cautioned against over optimism.

"I think it's important for us to stay focused and not get too optimistic but to try to appreciate the difficulties we have and see if we can get through them," he said, after having dinner with his North Korean counterpart Kim Kye-gwan.

North Korea shut down and sealed its Soviet-era Yongbyon nuclear plant and allowed U.N. atomic energy monitors back to the site in July, following the February 13 six-party deal.

In return, Pyongyang has received shiploads of heavy fuel oil and held bilateral talks with the United States that could bring the impoverished fortress state out of diplomatic isolation.

On Monday, U.S. Secretary of State Condoleezza Rice hinted that North Korea could be dropped from a U.S. terrorism blacklist before fully accounting for the Japanese citizens it abducted in the 1970s and 1980s, a move that could upset Japan.

But Hill did not make it clear whether Washington would move ahead to remove North Korea from the blacklist.

"It think the abduction issue is very important to Japan, a centre-piece of the bilateral talks that Japan has," Hill said earlier in Tokyo. "I don't think we are going to be able to achieve our goals in this entire process unless we have some real progress in the Japan-DPRK relationship."

The fate of Japanese citizens abducted by North Korea decades ago is a highly emotive issue in Japan.

North Korea admitted in 2002 that its agents had abducted 13 Japanese, sparking outrage in Japan.

Five of them were repatriated that same year, but Pyongyang says the other eight are dead. Tokyo wants more information about the eight and four others it says were also kidnapped, and wants any survivors sent home.

A failure to improve ties could spoil the six-party agreement because Tokyo is reluctant to give large-scale aid to Pyongyang in return for abandoning its nuclear ambitions.