US nuclear envoy Christopher Hill has expressed optimism after meeting his North Korean counterpart Kim Kye-gwan.
Christopher Hill hopes to commit Pyongyang to a timetable
He said both sides were "in the same ballpark" and held good discussions.
The meeting took place in Beijing on the eve of another round of six-nation talks, aimed at getting Pyongyang to give up its nuclear weapons programme.
North Korea shut down its main nuclear reactor on Saturday. Mr Hill said his talks with Mr Kim focused on setting a timetable for the next stage.
Such a phase would involve the listing of all of North Korea's nuclear facilities and eventually disabling the Yongbyon reactor completely.
"My idea is that we try to wrap this up" by the end of the year, he told reporters after meeting Mr Kim.
Before leaving for Beijing, Mr Kim said the six-party talks would address "obligations and actions" to be taken by all sides now that Pyongyang has shut down its reactor at Yongbyon.
The shutdown was the first step in a deal agreed in February, under which Pyongyang agreed to end its nuclear programme in return for aid.
On Monday, International Atomic Energy Agency chief Mohamed ElBaradei confirmed that a 10-man team of monitors at Yongbyon had verified the shutdown of the plutonium-making reactor.
He said it was "a good step in the right direction", but warned that there was still a long way to go.
N KOREA NUCLEAR DEAL
N Korea to "shut down and seal" Yongbyon reactor, then disable all nuclear facilities
In return, will be given 1m tons of heavy fuel oil
N Korea to invite IAEA back to monitor deal
Under earlier 2005 deal, N Korea agreed to end nuclear programme and return to non-proliferation treaty
N Korea's demand for light water reactor to be discussed at "appropriate time"
Under the February deal, North Korea is to receive 50,000 tons of energy aid in return for shutting Yongbyon down. Two shipments of aid have already been sent.
North Korea will then receive a further 950,000 tons of aid if it disables all its nuclear facilities.
But experts say that persuading North Korea to come clean about all of its nuclear activities and agree to their being disabled is likely to be a far more complicated process.
"I think you have to look at each stage as more difficult than the previous stage," Mr Hill said on Monday in the South Korean capital, Seoul.
"It is a little like one of those video games - every level becomes more difficult than the previous level."
North Korea - which carried out its first nuclear test in October 2006 - continues to deny US allegations that it has a secret uranium enrichment programme.