North Korea's return to the nuclear disarmament process is an essential next step before any discussions about political and economic concessions can begin, a senior U.S. official said Wednesday.

The nuclear-armed North, which walked away from the disarmament negotiations last year, wants a peace treaty to formally end the Korean War as well as diplomatic relations with the U.S. before giving up its nuclear ambitions.

Pyongyang cites the U.S. military presence in South Korea as its main reason for building a nuclear weapons program. North Korea is believed to have enough weaponized plutonium for at least half a dozen nuclear bombs, and last year revealed it has a uranium enrichment program that would give it a second way to make atomic weapons.

Assistant Secretary of State Kurt Campbell told reporters in Seoul that getting the talks back on track is a top priority. He was in Tokyo earlier in the week and will be in Seoul through Thursday to strategize with Washington's allies. The talks involved six nations: the two Koreas, the U.S., Japan, Russia and China.

"It is essential ... to make very clear to our North Korean interlocutors that the essential next step is really the six-party talks, not discussions on other matters," he said. "It's possible to have discussions on other matters within the six-party framework. But that six-party framework is essential going forward."

No discussion about easing international sanctions, a peace treaty or diplomatic relations can take place before the disarmament talks are back on track, Campbell said. North Korea must also prove it will honor previous commitments to disarm, he told South Korean reporters earlier Wednesday, according to the Yonhap news agency.

Communist North Korea has twice tested nuclear bombs, in 2006 and last May, and test-fired long-range missile in defiance of U.N. Security Council resolutions barring the regime from developing its atomic weapons and ballistic missiles programs.