Speaking on a visit to Japan, Secretary-General Guterres said: "The worst possible thing that could happen would be for us all to sleepwalk into a war that might have very dramatic circumstances."
The UN Security Council has imposed three rounds of sanctions over the past year against North Korea over its increasingly powerful missile and nuclear tests.
Guterres said those sanctions need to be implemented "by North Korea first of all, but also fully implemented by all the other countries whose role is crucial."
He urged "diplomatic engagement that allows for ... denuclearisation (of the Korean peninsula) to take place in a peaceful way."
Speaking alongside Guterres, Japanese Prime Minister Shinzo Abe echoed the call to "fully implement United Nations Security Council resolutions" and hold "meaningful dialogue toward denuclearisation."
The secretary-general's trip comes after Jeffrey Feltman, the UN's political affairs chief, visited North Korea earlier this month.
On Tuesday, US Secretary of State Rex Tillerson said Washington was ready to talk to Pyongyang "without preconditions", interpreted by many as a shift in the US position.
Washington's top diplomat told the Atlantic Council policy forum that North Korea is welcome to talk about anything at a first meeting -- even the weather.
However, the White House and Tillerson's own department later stressed that the US position on North Korea "had not changed."
"The secretary was not creating a new policy, our policy remains exactly the same as it was," State Department spokeswoman Heather Nauert said Wednesday.
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