US Secretary of State Rex Tillerson has drawn a distinction between “talks” with North Korea and “negotiations”, arguing Donald Trump’s willingness to chat with Kim Jong-un should not be construed as anything more.

The stunning announcement that Trump had agreed to a meeting with the North Korean leader raised questions about what had changed after months of Tillerson and other Trump officials insisting the conditions were not right for negotiations with Pyongyang.

Tillerson said 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,” said Tillerson.

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.

Explaining Trump’s decision-making about the meeting, 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.

"What changed was his posture in a fairly dramatic way," Tillerson said.

"In all honesty, that came as a little bit of a surprise to us as well."

As Trump's administration ramped up its "maximum pressure campaign" on North Korea over the last year, 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, Trump decided Kim's willingness to discuss denuclearisation and commit to halt testing was enough.

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," 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."