US nuclear envoy Christopher Hill has said North Korea should be treated with patience, days after a key deadline for declaring its nuclear plans was missed.
Mr Hill is in Japan on the first leg of a four-nation tour
He said a late declaration was preferable to an incomplete one, stressing that the US was ready to persevere with Pyongyang.
Mr Hill spoke in Tokyo at the start of a regional tour, aimed at moving forward the denuclearisation process.
Earlier, North Korea had agreed to end its programme in return for aid.
Officials have already begun to disable its main reactor at Yongbyon, under the supervision of international experts.
But Pyongyang missed a year-end deadline to provide a complete account of its nuclear activities.
The communist regime hit back at criticism of its missed deadline, accusing the US and other countries of being slow to deliver aid.
Arriving in Tokyo, Mr Hill said that the quality of North Korea's nuclear declaration outweighed the timing.
N Korea to "shut down and seal" Yongbyon reactor, then disable all nuclear facilities
In return, it will be given 1m tons of heavy fuel oil
Under an earlier 2005 deal, N Korea agreed to end nuclear programme and return to non-proliferation treaty
N Korea's demand for a light water reactor to be discussed at an "appropriate time"
"They were prepared to give a declaration which wasn't going to be complete and correct and we felt that it was better for them to give us a complete one even if it's going to be a late one," he told journalists.
"We understand that this is always a difficult process, one that is rarely completed on time. So I think we have to have a little sense of patience and perseverance."
The US wants to know how much plutonium is in the hands of the communist state, which tested a nuclear bomb in October 2006.
There is also the issue of whether or not Pyongyang has a secret uranium enrichment programme - something the US believes but which North Korea denies.
Mr Hill said that during his four-nation tour he would "compare notes" with fellow negotiators over where the talks with North Korea should go.