The US envoy for North Korea has returned from Pyongyang, where he held talks to prepare for a second US-North Korean leadership summit.
Stephen Biegun spent three days in discussions, and US officials said he would meet his counterpart again.
US President Donald Trump confirmed he would meet North Korea’s Kim Jong-un in Hanoi, Vietnam, on 27-28 February.
Mr Trump said his representatives had a “productive meeting” and that he was looking forward to advancing peace.
How did Mr Biegun’s visit go?
In Pyongyang, Mr Biegun met his North Korean counterpart Kim Hyok-chol and “discussed advancing President Trump and Chairman Kim’s Singapore summit commitments of complete denuclearisation,” said a US State Department statement.
The two envoys will meet again ahead of the much anticipated summit at the end of the month.
On his return to South Korea, Mr Biegun briefed the country’s Foreign Minister Kang Kyung-wha.
“I am confident that if both sides stay committed, we can make real progress,” he told reporters.
“We have some hard work to do with the DPRK [North Korea] between now and then,” Mr Biegun added, looking ahead to the Hanoi summit.
President Trump tweeted that “North Korea, under the leadership of Kim Jong Un, will become a great Economic Powerhouse,” something that, the president added, came as no surprise to him “because I have gotten to know him & fully understand how capable he is”.
Meanwhile, the UN has warned that North Korea is continuing its nuclear programme and breaking sanctions.
In a report earlier this week, it 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 first summit between President Trump and Kim Jong-un in Singapore last June 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.
Before travelling to Pyongyang on Wednesday, Mr Biegun said he wanted to achieve some “concrete deliverables”.
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”.