US Secretary of State Mike Pompeo on Tuesday abruptly shelved plans to meet a top North Korean official in New York, the latest twist in diplomacy to secure a potentially landmark peace deal.
"We will reconvene when our respective schedules permit," she added in a statement.
State Department officials gave no further reasons for the delay in the meeting, although North Korea has stepped up its demands that the United States lift sanctions.
The State Department had just one day earlier confirmed that Pompeo would meet in New York with Kim Yong Chol -- one of the North Korean leader's right-hand men -- to discuss progress toward a denuclearization pact and to work to arrange a second summit following landmark talks between Trump and Kim in June.
Although Trump has had warm words since he meet with Kim in Singapore, his administration has nevertheless insisted on maintaining pressure on North Korea until a final agreement is reached.
North Korea said last week that it would "seriously" consider returning to a state policy aimed at building nuclear weapons unless Washington lifts sanctions.
"The improvement of relations and sanctions are incompatible," said a statement carried by the official Korean Central News Agency.
"What remains to be done is the US corresponding reply," it added.
Pompeo, speaking Sunday on Fox News, said he was "not worried" about the North Korean demands and insisted there would be "no economic relief until we have achieved our ultimate objective."
"I expect we'll make some real progress, including an effort to make sure that the summit between our two leaders can take place where we can make substantial steps toward denuclearization," Pompeo said separately Sunday on CBS's "Face the Nation."
Pompeo has traveled four times this year to North Korea, for decades a US pariah, in hopes of securing an accord. The diplomacy comes a year after fears mounted of war, with Trump threatening "fire and fury" after Pyongyang's nuclear and missile tests.
Trump has cast North Korea as a crowning diplomatic achievement and is eager for a fresh summit with Kim at which the two may formally declare an end to the 1950-53 Korean War.
North Korea has long sought US recognition as a nuclear state and guarantees for the survival of the generational Kim regime, which human rights groups consider to be one of the most repressive in the world.
Critics in the United States say that North Korea has yet to make any concrete concessions.
Kim Yong Chol is a general, a former top intelligence chief and right-hand man to the North Korean leader. He visited New York in May for talks with Pompeo in what was the highest-level trip by a North Korean to the United States in nearly two decades.
The diplomacy on North Korea has had a series of fits and starts, with Trump at one point scrapping a trip by Pompeo just as he was set to fly off to Pyongyang.
The latest New York meeting was set to come ahead of a busy season of diplomacy, with Pompeo meeting senior Chinese officials in Washington on Friday.
Trump heads over the next month to international gatherings in Paris and Buenos Aires, while Vice President Mike Pence will tour Asia.