US Secretary of State Mike Pompeo abruptly shelved plans to meet a top North Korean official in New York on Tuesday, the latest twist in diplomatic efforts to secure a potentially landmark peace deal.
The talks between President Donald Trump's top diplomat and the North Korean delegation, which had been due on Thursday, "will now take place at a later date," State Department spokeswoman Heather Nauert said.
"We will reconvene when our respective schedules permit," she said in a statement.
State Department officials gave no reason for the delay, although North Korea has stepped up its demands that America lift sanctions on Pyongyang.
The State Department had just one day earlier confirmed that Mr Pompeo would meet in New York Kim Yong Chol — one of the North Korean leader's right-hand men — to discuss progress towards a denuclearisation pact and to work to arrange a second summit following historic talks between Mr Trump and Mr Kim in June.
Despite warm words from Mr Trump since meeting Mr Kim in Singapore, his administration has insisted on maintaining pressure on Pyongyang 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 said.
Mr Pompeo, speaking on 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".
But Yang Moo-jin, a professor at the University of North Korean Studies in Seoul, said that the postponement was a bad sign.
"This last-minute announcement of a delay is not a good signal as it indicates negotiations were not going well enough to go ahead with the planned meeting," he said.
Mr Pompeo has this year made four trips to North Korea, for decades a US pariah, in the hope of securing an accord.
The diplomacy comes a year after fears of war mounted when Mr Trump threatened "fire and fury" after Pyongyang's nuclear and missile tests.
Mr Trump has cast North Korea as a crowning diplomatic achievement and is eager for a fresh summit with Mr 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 one of the most repressive in the world.
US critics say that North Korea has yet to make any concrete concessions and it has rejected demands for what it described as its "unilateral disarmament".
Mr Pompeo's postponement of talks came amid emerging differences between Seoul and Washington, which stations 28,500 troops in the South.
In contrast to Washington's approach, the South's dovish president Moon Jae-in, who brokered the Singapore summit, has dangled large investment and joint cross-border projects as incentives for steps towards denuclearisation.
South Korea's foreign ministry said the meeting delay was "regrettable" but insisted that "excessively pessimistic interpretation was unnecessary".
"There have been cases in the past when US-North Korea talks were delayed, so there is no need to shift between hope and fear," a senior ministry official said.
Kim Yong Chol is a general and a former top intelligence chief. He visited New York in May for talks with Mr Pompeo in what was the highest-level trip by a North Korean to the US in nearly two decades.
The diplomacy on North Korea has been troubled by a series of fits and starts, with Mr Trump at one point scrapping a trip by Mr Pompeo just as he was set to fly to Pyongyang.
The latest New York meeting was set to come before a busy season of diplomacy, with Mr Pompeo meeting senior Chinese officials in Washington on Friday.
Next month, Mr Trump is due to attend international gatherings in Paris and Buenos Aires, while Vice President Mike Pence will tour Asia.