"What changed was his posture in a fairly dramatic way," Tillerson said in Djibouti during a five-nation Africa tour.
"In all honesty, that came as a little bit of a surprise to us, as well that he was so forward-leaning in his conversations with the delegation from South Korea."
Tillerson's comments came after President Trump agreed to a historic meeting with Kim, the first ever between a US president and a North Korean leader.
The meeting is due to take place by the end of May.
News of the summit, which South Korean President Moon Jae-in said was "like a miracle", is the latest step in a quickening detente after a period of heightened rhetoric and extreme tension between Washington and Pyongyang.
"In terms of direct talks we're a long way from negotiations, we just need to be very clear-eyed and realistic about it," Tillerson had said in the Ethiopian capital of Addis Ababa on Thursday.
A first step would be "talks about talks" to see if "conditions are right to even begin thinking about negotiations," Tillerson had said.
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