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".
'Concrete deliverables'
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./BBC