The United States' chief negotiator with North Korea, Christopher Hill, is to visit the country's nuclear complex at Yongbyon.
Mr Hill said it was time to move to the next phase of negotiations
Mr Hill, who is in North Korea until Wednesday, will be the highest-level US diplomat to visit the site.
He will inspect the continuing work to disable the facility and render it unable to produce nuclear bombs.
In October Pyongyang agreed to end its nuclear programme in exchange for aid and diplomatic concessions.
The disabling of the Yongbyon plant, about 100km (60 miles) north of Pyongyang, is a major step in North Korea's pledge to cease production of weapons-grade plutonium by the end of the year.
The process of removing 8,000 fuel rods from the nuclear reactor began last month under the guidance of US experts.
Mr Hill said the disabling of the plant and a full declaration of North Korea's nuclear programme would be "a very dramatic improvement in the situation on the Korean peninsula".
It would begin a process of bringing North Korea into the international community and "completing normalisation of US ties," he added.
Mr Hill will also meet North Korea's top nuclear Kim Kye-gwan, during his visit.
He told reporters he would be briefing Mr Kim on what Washington is doing to fulfil its responsibilities under the aid-for-disarmament agreement.
As well as inspecting Yongbyon, Mr Hill is planning to discuss North Korea's pledge to openly declare all its nuclear programmes by the end of this year.
I am "looking forward to seeing the declaration soon - the declaration of all their nuclear programmes, materials and infrastructure," Mr Hill said.
If North Korea complies with all requirements, the US has promised to remove it from a list of countries it says support terrorism, as well providing as an extensive aid package.