One of North Korea's top negotiators is on his way to Washington to meet US officials ahead of a possible second summit between Kim Jong-un and US President Donald Trump.
South Korean reports say that Kim Yong-chol, flying from Beijing, is carrying another letter from Mr Kim to Mr Trump.
He is expected to meet US Secretary of State Mike Pompeo on Friday, South Korea's Yonhap news agency said.
Speculation is mounting that the second summit could be held in Vietnam.
The North Korean leader is scheduled to travel to the communist-run country for an "official state visit" sometime in February, Reuters news agency is reporting, citing unnamed sources.
Little progress has been made on North Korean de-nuclearisation since a historic meeting in Singapore last June between Mr Kim and Mr Trump.
But there has recently been a flurry of diplomatic activity. Mr Kim visited China earlier this month for talks with President Xi Jinping, as he did before the Singapore summit.
Meanwhile another senior North Korean official, vice-foreign minister Choe Son-hui, is heading to Sweden, where she could meet Washington's special representative for Pyongyang Stephen Biegun, reports say.
Who is Kim Yong-chol?
General Kim Yong-chol, a former spymaster often referred to as Kim Jong-un's right hand man, has emerged as North Korea's lead negotiator in recent talks with the US.
He's a controversial figure, and has been accused of masterminding attacks on South Korean warships during his time as military intelligence chief in 2010.
He last visited Washington in June, when he delivered a letter to Mr Trump ahead of historic talks between both countries.
What could these talks achieve?
It's not clear. The last time Mr Kim went to the US, his letter to Mr Trump appeared to have helped get the Singapore summit back on track.
Negotiations between both countries have stalled since then, but this meeting could be what it takes to restart talks.
Earlier this month Mr Trump said the US and North Korea were negotiating over a location for another summit but US officials have not provided any further details.
The meetings in Washington could finalise plans for the summit, but just as important, analysts say, would be an understanding of what the agenda would be.
In his annual new year's speech a few weeks ago, Mr Kim said he was committed to denuclearisation, but warned that he would change course if US sanctions remained.
Both parties signed a pledge in Singapore to denuclearise the Korean peninsula, though it was never clear what this would entail.
- BBC / Reuters