The US envoy for North Korea has been holding talks in Pyongyang, paving the way for a second leadership summit.
Stephen Biegun arrived just as US President Donald Trump confirmed he would meet North Korea's Kim Jong-un in Vietnam on 27-28 February.
The two leaders will build on the vague denuclearisation commitments they made when they met in Singapore last June.
Meanwhile, the UN has warned that North Korea is continuing its nuclear programme and breaking sanctions.
The report said actions including the illegal transfer of banned goods at sea could make sanctions - the international community's main way of putting pressure on North Korea - "ineffective".
The Singapore summit generated significant coverage and optimism, but delivered very few concrete developments.
Both sides said they were committed to denuclearisation, but with no details of how this would be carried out or verified.
Experts caution that despite Mr Trump's declaration that North Korea is no longer a nuclear threat, the country has never said it would give up its nuclear weapons programme without similar concessions from the US.
After holding talks with officials in South Korea, Mr Biegun travelled to Pyongyang to talk with his North Korean counterpart, Kim Hyok-chol.
Mr Biegun said he wanted to achieve some "concrete deliverables".
The US state department has said his visit will "advance further progress on the commitments the president and Chairman Kim made in Singapore".
The US wants North Korea to make a full declaration of all its nuclear weapons facilities and commit to destroying them, under international supervision - something North Korea has never said it will do.
In a speech at Stanford University last week, Mr Biegun said the US would not agree to lift sanctions until this happens, but he indicated it could provide assistance in other ways, saying: "We did not say we will not do anything until you do everything."
He also said Kim Jong-un had previously committed to "the dismantlement and destruction" of all North Korea's plutonium and uranium facilities, which provide the material for nuclear weapons.
But a report to the UN Security Council on Monday suggested North Korea was continuing its nuclear and missiles programmes, while making efforts to protect its facilities from possible future strike.
The confidential report, a copy of which was seen by news agencies, also said North Korea was routinely breaking international sanctions.
The report said there had been a "massive increase in illegal ship-to-ship transfers of petroleum products and coal" - where material is moved from non-North Korean ships out at sea to evade monitoring.
The international sanctions against North Korea are designed to severely limit its import and export abilities, with the aim of putting pressure on the country to give up its nuclear ambitions.
But Reuters quoted the report as saying that violations on this scale "render the latest UN sanctions ineffective".
Is the main diplomacy tool ineffective?
Laura Bicker, BBC News, Seoul
This UN report states what to many is blindingly obvious - North Korea's nuclear and ballistic missile programmes "remain intact" despite the "tremendous progress" hailed by the Trump administration.
But Pyongyang's pledges, which we have heard through either the South Koreans or the Americans, were very specific. They said they would destroy a nuclear testing site and dismantle a rocket launch site.
The North Koreans have at no point said they will hand over all their weapons nor that they would stop building them. This is why getting a detailed deal is so important.
North Korea has, through the South Korean president, pledged to destroy a nuclear processing plant, but only if the US takes corresponding steps. We expect this to be the focus of at least some of the talks between the two sides over the next few weeks.
What is interesting about this report, and perhaps more worrying for the Trump administration, is that their main diplomatic tool to encourage Kim Jong-un to get to the negotiating table may now be "ineffective".
There have been many reports of illegal ship-to-ship transfers of oil and coal over the last year, but sanctions monitors now say there is a "massive increase".
If Pyongyang is finding a way around these strict sanctions, then it means Washington's maximum pressure strategy is never going to work.
We are entering a pivotal month in this peninsula's future and this report highlights the challenges facing the Trump administration. It also raises the question, that if these talks fail, and sanctions are no longer effective, what does the US do next?