Christopher Hill, US assistant secretary of state for East Asia and the Pacific, wrapped up talks in Tokyo at the end of a quick three-nation tour to pave the way for a visit next week by US Secretary of State Condoleezza Rice.

"We have a lot of work ahead of us, but I think it is important to remember all the work we have been able to accomplish," he told reporters after meeting with his new Japanese counterpart Akitaka Saiki.

"We'll look at the work we were able to accomplish in the past and I think we can take from that some strength and some sense of optimism," he said.

"Obviously we are having some difficulty right now in getting what we need, which is a complete and correct declaration," he added.

North Korea last year signed a breakthrough six-nation deal to abandon its nuclear weapons in exchange for badly needed aid and security guarantees.

The administration of US President George W. Bush, who once branded North Korea part of an "axis of evil," has promoted the deal as a key foreign policy achievement as it prepares to leave office.

But the North missed a deadline to declare all of its nuclear programmes by the end of last year amid US allegations it has a covert uranium enrichment project in addition to its declared plutonium.

Kim Kye-Gwan, the North Korean chief envoy to the six-nation talks, held talks Thursday in Beijing with his counterpart from South Korea, which has supported reconciliation with its communist neighbour.

"I don't think the gaps are too wide to bridge. We only need more time and efforts. I believe the North is also willing to resolve this issue," South Korean chief envoy Chun Yung-Woo said after the talks.

Hill also held talks earlier this week in Beijing with Kim, who again rejected US calls to declare alleged uranium enrichment.

The February 2007 deal was reached after protracted negotiations by the six nations -- China, Japan, the two Koreas, Russia and the United States.

Japan has been the most critical member of the six-nation forum due largely to a dispute over North Korea's past kidnappings of Japanese civilians.

A senior Japanese official, speaking on condition of anonymity, said that Hill, despite his optimism, was taking a tough line on securing a declaration of the North's atomic programmes.

"His tone has become tougher. That could mean that President Bush has instructed him to press North Korea for a complete declaration," the Japanese official said.

Under the February agreement, the North was to receive one million tonnes of fuel oil or equivalent energy aid in return for disablement and a declaration.

The six-nation agreement also set in motion a process to establish diplomatic relations between the United States and North Korea, which technically only suspended and never ended the 1950-53 Korean War.

In a sign of reconciliation, the New York Philharmonic orchestra will on Tuesday perform in Pyongyang with the musical selection to include Antonin Dvorak's ninth symphony, "From the New World."

Hill played down speculation that he or Rice would visit Pyongyang for the concert. Both will be in Seoul on Monday for President-elect Lee Myung-Bak's inauguration.

"I'm going to be at the inauguration of the (South Korean) president," Hill said. "This happens to be within 24 hours, so I'll be in South Korea, not in North Korea."

Rice "will be in South Korea as well," Hill said.

The Pyongyang concert is already being compared to US orchestral visits to the Soviet Union in the 1950s and "ping pong" diplomacy with China in the 1970s.

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