"We believe it's high time to make sure that the negotiations between the United States and the Democratic People's Republic of Korea start again seriously and that a roadmap is clearly defined for the denuclearization of the Korean peninsula," Guterres told a news conference.

His remarks came as top North Korean envoy Kim Yong Chol was in Washington for talks with President Donald Trump and Secretary of State Mike Pompeo to prepare a second summit, probably in Vietnam.

Trump and North Korean leader Kim Jong Un held a historic summit in June in Singapore -- the first ever between the two countries -- where they signed a vaguely worded denuclearization deal.

But there has been no progress made since then, with both sides disagreeing over what was signed in Singapore.

Asked about a possible easing of sanctions, Guterres said he would not "advocate for any anticipation of other measures before a clear negotiation is put in place" but added that "the two things will be inevitably interlinked."

The United States has insisted that "maximum pressure" from a raft of tough sanctions imposed on North Korea by the Security Council must remain in place until Pyongyang has fully scrapped its nuclear and ballistic missiles program.

But North Korea insists that easing sanctions would build trust along the way to denuclearization - a stance that has won support from China and Russia.

Guterres also said humanitarian aid to North Korea should not be held hostage to political considerations.

The United States this month agreed to begin lifting a hold on several requests to the UN sanctions committee from aid agencies for exemptions that would allow them to ship goods needed for relief work in North Korea.

"We should never refuse humanitarian aid to any country in any circumstance, for the people of that country," said Guterres.

The UN chief said the United Nations would not seek to play a bigger role in efforts to denuclearize the Korean peninsula, leaving the diplomacy firmly in the hands of Washington and Pyongyang.