The U.S. Secretary of State, Mike Pompeo, is expected to meet with a North Korean official this week, according to new reports on Sunday.
The talks between President Donald Trump's top diplomat and the North Korean delegation were due to be held on Thursday.
It offered no reason and North Korea's propaganda services have not mentioned the meeting.
Nauert said the U.S. remained focussed on fulfilling the commitments agreed to by President Donald Trump and North Korean leader Kim Jong-un at the Singapore summit in June, reported The Washington Post.
A Thursday meeting between Secretary of State Mike Pompeo and his North Korean counterpart in NY has been abruptly called off without explanation in another setback for the Trump administration's nuclear negotiations. Pompeo has travelled four times this year to North Korea to work on achieving an accord.
Pompeo was supposed to travel to NY on Thursday to meet with his North Korean counterpart, Kim Yong Chol.
South Korea, however, said the delay does not mean the summit won't happen or "momentum for the summit has been lost", reported Reuters, quoting presidential spokesperson Kim Eui-kyeo.
North Korea has been pushing for such a declaration before it takes any further steps toward denuclearization, but Washington fears that this would rob it of a powerful negotiating card.
But it comes amid renewed doubts about the Trump administration's efforts to convince North Korea to abandon its nuclear arsenal in exchange for security guarantees and sanctions relief.
North Korea and the United States are finally going to be sitting down for high-level talks - some five months after their summit in Singapore.
South Korea's presidential Blue House said the United States had informed it of the talks' postponement in advance.
"Gen. Dunford was simply highlighting that the Defense Department is in support of the State Department when it comes to the ongoing diplomatic negotiations with North Korea", JCS spokesperson Col. Patrick S. Ryder told Voice of America.
North Korean leader Kim Jong-un told President Moon Jae-in during a summit that submitting an inventory of his country's nuclear arsenal and facilities to the US could spark an attack, national security adviser Chung Eui-yong said Tuesday.
Trump's engagement with North Korea in recent months was apparently timed to the midterms, but he will no longer agree to major meetings if there are no realistic expectations of progress in nuclear diplomacy, Koh said.