Fredrik Reinfeldt, who led Sweden with a centre-right government between 2006 and 2014, compared Mr Trump with a “testosterone-fuelled teenager” whose mocking of Kim had done little to help soothe rising tensions in the region.
Speaking on Aftonbladet TV in an interview looking back on 2017, he described the crisis on the Korea Peninsula as “one of the biggest threats of war we have in the world”.
When asked if he thought there was a genuine risk of nuclear war as a result of the regular swapping of insults, Mr Reinfeldt said: “Yes, in the sense that North Korea continues to launch missiles and soon one will land on the wrong place.
“The time it doesn’t land in the water, the person that is hit by it will hit back. There’s absolutely a risk of it”.
The 52-year-old said Mr Trump’s lack of diplomacy was an issue.
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He said: “He’s like a teenage boy who always has to answer with more testosterone.
“That’s rarely the right answer and in this case clearly dangerous.”
This is not the first time Mr Reinfeldt has raised the alarm over the US President.
In August 2016 he warned the “emotionally volatile” approach of the New York businessman would make the world a more dangerous place if he won the US election.
Mr Reinfeldt ominous warning came as Kim has declared North Korea’s nuclear capabilities now posed a “substantial threat” to the US.
A North Korean propaganda outlet said: “Kim Jong-un stressed that nobody can deny the entity of the DPRK which rapidly emerged as a strategic state capable of posing a substantial nuclear threat to the US.”
The fiery comment was made at a Korean Workers Party conference of cell chairpersons which co-ordinate the parties lowest level members.
The outburst was yet another volley in the war of words with Mr Trump that has become a hallmark of the dangerous tensions between Pyongyang and the US highlighted by Mr Reinfeldt.