Update 12.51pm: US Secretary of State Rex Tillerson has drawn a distinction between "talks" with North Korea and "negotiations", arguing President Donald Trump's willingness to chat with Kim Jong Un should not be construed as anything more than that.
The stunning announcement that Mr Trump had agreed to a meeting with the North Korean leader raised questions about what had changed after months of Mr Tillerson and other Trump officials insisting the conditions were not right for negotiations with Pyongyang.
Mr Tillerson said Mr Trump has been open to mere talks and a meeting with Kim "for some time", and had decided "the time was right".
"In the president's judgment, that time has arrived now," Mr Tillerson told reporters in Djibouti during a trip to Africa.
Mr Tillerson did not define the precise difference between talks and negotiations, and it was unclear what there was for the two countries, still technically at war, would have to discuss if not a deal to address concerns about the North's nuclear weapons programme.
Ostensibly, they could hold preliminary conversations to see if there's enough common ground and good will to proceed to formal negotiations.
Explaining Mr Trump's decision-making about the meeting, Mr Tillerson said that the US had witnessed a shift from North Korea that became apparent when a South Korean delegation visited Pyongyang, then travelled to Washington to brief US officials on the rare meeting.
He said the dispatch from that meeting "was the most forward-leaning report that we've had, in terms of Kim Jong Un's not just willingness but his strong desire for talks.
As Mr Trump's administration ramped up its "maximum pressure campaign" on North Korea over the last year, Mr Tillerson was one of the more enthusiastic advocates within the Cabinet for trying to talk to the North Koreans, even as other officials warned Trump of the risks of rewarding Kim too soon.
For months the administration gave mixed messages about just what "preconditions", if any, needed to be met to merit talks.
Ultimately, Mr Trump decided Mr Kim's willingness to discuss denuclearisation and commit to halt testing was enough.
Mr Tillerson said the decision to agree to the meeting was "a decision the president took himself".
"This is something he's had on his mind for quite some time," Mr Tillerson said.
"So now I think it's a question of agreeing on the timing of that first meeting between the two of them and a location and that will take some weeks before we get all that worked out."
Earlier: US President Donald Trump and North Korea leader Kim Jong Un to meet
US President Donald Trump has decided he will meet with North Korea's Kim Jong Un "by May", South Korea's national security director has said.
Chung Eui-yong said Mr Trump said "he would meet Kim Jong Un by May to achieve permanent denuclearisation" of the Korean peninsula.
The security director said Mr Kim told the South Koreans he is "committed to denuclearisation" and pledged that "North Korea will refrain from any further nuclear or missile tests".
The meeting would be the first of its kind between a leader of North Korea and a sitting president of the United States.
Earlier in a tweet, Donald Trump said "great progress" was being made, although sanctions will remain in place until an agreement is reached.
He said: "Kim Jong Un talked about denuclearization with the South Korean Representatives, not just a freeze.
"Also, no missile testing by North Korea during this period of time. Great progress being made but sanctions will remain until an agreement is reached. Meeting being planned!"
Kim Jong Un talked about denuclearization with the South Korean Representatives, not just a freeze. Also, no missile testing by North Korea during this period of time. Great progress being made but sanctions will remain until an agreement is reached. Meeting being planned!— Donald J. Trump (@realDonaldTrump) March 9, 2018
The latest round of sanctions to be imposed on North Korea by the US was announced on Wednesday, when the State Department revealed punitive measures in response to its finding that Pyongyang was behind the nerve agent assassination of Mr Kim's half-brother.
South Korean President Moon Jae-in said the planned summit between the two leaders will be a "historical milestone" that will put the denuclearisation of the Korean Peninsula "really on track".
Mr Moon in a statement read out by his spokesman also complimented Mr Trump for accepting Mr Kim's invitation for a summit, saying the president's leadership will be praised "not only by the residents of South and North Korea but every peace-loving person around the world".
Mr Moon is also preparing for a summit with Mr Kim at a border village between the Koreas in April.
- Press Association and Digital Desk