"All world leaders agree that he [Kim Jong Un] is a unsafe man", Duterte told a press conference before leaving for a two-day visit to Japan.
"Somebody has to go there".
Duterte, who is fond of lengthy, freewheeling speeches, said he would skip parts of his statement because he was hungry.
The Philippine president, who is notorious for his short temper and abusive language against his critics, had previously described Kim as a "fool" and "son of a bitch" for "playing with unsafe toys".
"We are anxious. If anything can go wrong, it will go wrong", he said. "A nuclear war is totally unacceptable to everybody", Duterte stressed, saying the global community needed to assure Kim that "nobody's threatening him".
Duterte said he would also discuss with Abe the possible solutions to stop North Korea's Leader Kim Jong-Un from making provocative actions. "I expect to be dealing with him on these topics, with the situation in the Korean peninsula being the main agenda", Mr Duterte said in a media briefing before leaving for Japan to meet Prime Minister Shinzo Abe.More news: State police urge trick or treat safety
United States expert Yasushi Watanabe of Keio University noted that while this appeared to have taken a back seat over North Korea, the U.S. has demonstrated its commitment to the region by holding four freedom of navigation operations in the South China Sea over five months.
Another highlight in Duterte's second visit to Japan was his meeting with Japanese Emperor Akihito and wife Empress Michiko at the historic Imperial Hotel.
In their meeting held at the prime minister's office in Tokyo, Abe and Duterte shared the view that North Korea's nuclear and missile development programs pose a threat.
Duterte's visit comes comes a week before US President Donald Trump's trip to Asia, starting in Japan and continuing on to South Korea, China, Vietnam and the Philippines.
The official visit also saw Filipino business leaders striking $6 billion worth of deals with Japanese firms. "I dare say that, considering what we have accomplished and achieved, the Philippines and Japan are building a golden age (for) our strategic partnership", Duterte said.
Under Duterte's presidency, Manila has drifted closer to Beijing, with the Philippine government announcing a "separation" from Washington a year ago.