South Korea and the United States are wrapping up senior diplomatic talks on North Korea's nuclear weapons with a show of unity

They say a six-nation process that has been underway for six years must be preserved.

Senior U.S. Envoy on North Korea, Stephen Bosworth, emerged from meetings with South Korean counterparts in Seoul holding firm to Washington's insistence that Pyongyang's nuclear capabilities must be ended.

"We have had very useful conversations here with our South Korean partners," he said, "we are agreed entirely that denuclearization, complete and verifiable, remains our core interest."

Bosworth has been in the South Korean capital since Friday, holding meetings with officials including chief South Korean nuclear negotiator Wi Wung-lac and Unification Minister Hyun In-taek.

Both sides are expressing concern over North Korean statements on Friday that Pyongyang is continuing to extract plutonium from spent nuclear fuel for material useable in weapons.

The North also said it had reached the final stage of a uranium enrichment program, possibly creating a second process by which it can build nuclear arms. Until this year, Pyongyang has denied U.S. accusations it possessed a covert HEU, or highly enriched uranium program, in violation of previous agreements.

Bosworth said the problem will take time to solve.

"This is not the first we have heard of HEU," he said. "And it may not be the last."

Bosworth repeated the Obama administration's policy that Washington is willing to talk one-on-one with North Korea, but only within the framework of six-nation talks begun in 2003 aimed at ending all of the North's nuclear capabilities.

North Korea has declared those talks "dead" several times this year, and says it is willing to deal only with the United States as a fellow nuclear weapons nation.

The United States and South Korea say they will be diligent about implementing U.N. Security Council sanctions imposed after North Korea conducted the second nuclear weapons test in its history in May.