President Trump has tweeted that a deal with North Korea is “very much in the making”, a day after revealing he had agreed to meet its leader Kim Jong-un.
Earlier, the White House said the meeting would not take place unless Pyongyang took “concrete actions”.
US media report that Mr Trump made the decision to meet without consulting key figures in his administration, who are now scrambling to catch up.
No sitting US president has ever met a North Korean leader.
Mr Trump stunned observers when he agreed to the summit following an invitation delivered by South Korean envoys.
Confusion mounted when Mr Trump’s own press secretary, Sarah Sanders, told reporters that North Korea has “promised to denuclearise“. She added: “We’re not going to have this meeting take place until we see concrete actions.”
The top US diplomat, Secretary of State Rex Tillerson, was on his first official trip to Africa when the announcement was made.
He told reporters on Friday the decision to meet Mr Kim was one “the president took himself”.
“I spoke to him very early this morning about that decision and we had a very good conversation,” Mr Tillerson added.
On Saturday, Mr Tillerson cancelled all his official events in Kenya with aides saying the secretary was “not feeling well after a long couple days working on major issues back home such as North Korea”.
South Korean envoys – who recently met with Mr Kim in Pyongyang – have said North Korea is “committed to denuclearisation” as an end goal, but they have not said this would start before a meeting with the US.
Instead, North Korea is understood to have agreed to halt its testing programme as negotiations continues.