US envoy Christopher Hill has said North Korea is prepared to shut down its Yongbyon nuclear reactor promptly, following bilateral talks in Pyongyang.
Christopher Hill says there is hard work ahead
Mr Hill said North Korea had reaffirmed its commitment to fulfil an agreement on halting its nuclear programme, but that it would take time and effort.
He is the most senior US official to visit the secretive state since 2002.
North Korea agreed in February to shut down the reactor, but progress has been delayed by a row over frozen funds.
The US says the money has already been transferred from a Macau bank to North Korea, but Pyongyang has not yet confirmed this.
The confusion has thrown into doubt a visit by UN nuclear watchdog inspectors planned for next week - their first since they were forced out of the country in 2002.
Mr Hill held meetings with his North Korean counterpart, Kim Kye-gwan, and Foreign Minister Pak Ui-chun during his two-day visit.
Speaking at a press conference in the South Korean capital, Seoul, afterwards, Mr Hill said the talks had been detailed, substantive and useful.
"The DPRK - indeed both of us - reaffirmed our commitment to the February agreement and to the complete fulfilment of that February agreement," he said, referring to the North with an abbreviation of its official name, the Democratic People's Republic of Korea.
"The DPRK indicated that they are prepared, promptly, to shut down the Yongbyon facility."
The assistant secretary of state said that although the talks had been positive, he realised some parts of the agreement would take a long time to implement.
"I come away from this two-day set of meetings buoyed by a sense that we are going to be able to achieve our full objectives... but also burdened by the realisation of the fact that we are going to have to spend a great deal of time, a great deal of effort, a lot of work in achieving these," he said.
Mr Hill also said both parties had discussed getting away from issues such as the row over the frozen North Korean funds, and returning their focus to denuclearisation.
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"We also had a look ahead to what we have to do in the future to keep the process going and to really restore a sense of momentum and dynamism that will take us to the endgame," he said.
Mr Hill's visit had followed signs of visible progress, culminating last weekend in an invitation from Pyongyang for International Atomic Energy Agency (IAEA) inspectors to travel to the North to discuss shutting down Yongbyon.
The invite had come on the back of news that a Russian bank had accepted the transfer of $25m (£12.5m) of Pyongyang's funds, which had been frozen in a Macau bank since 2005.
Although Mr Hill said earlier this week he believed the money had been transferred, a North Korean diplomat said Thursday that completion of the move had not yet been confirmed - and warned it could mean a delay to the IAEA visit.