SEOUL — North Korea abruptly pulled out from a liaison office with South Korea near the border on Friday, another apparent sign of hardening positions following the no-deal summit between leader Kim Jong Un and President Trump last month.

North Korean staff members at the office left on “instructions from the superior authority,” according to South Korea’s Unification Ministry — an apparent reference to Kim.

North Korea’s withdrawal was a surprise but came amid toughening stances on all sides after Trump and Kim failed to make progress at talks in Hanoi.

The Trump administration has doubled down on its insistence that North Korea must make substantial steps to dismantle nuclear efforts before economic sanctions can be eased. Satellite images released earlier this month suggested North Korea has rebuilt rocket launch and test sites.

In South Korea, meanwhile, the North’s latest move sent a chill through South Korean President Moon Jae-in’s efforts for rapprochement with North Korea. It also cast doubts on the future of other cross-border initiatives, such as sports and cultural exchanges and linking the rail systems of the two Koreas.

“The North Koreans are masters of public pressure, and it’s clear they have decided that is what is necessary at the moment for both the United States and South Korea,” said Mintaro Oba, a speechwriter at West Wing Writers and a former diplomat at the State Department’s Korea desk.

Oba said the withdrawal from the office does not mean the “broader process” is falling apart and could be a tactic intended to put pressure on Seoul and Washington for concessions.

But Oba said there was also a risk that hard-liners are “taking the wheel” in Pyongyang after Hanoi, with the failure of that summit potentially undermining the domestic leverage of proponents of dialogue. “We don’t know the answers, but these are important potential factors to consider.”

Moon’s top national security adviser convened an emergency meeting to discuss the surprise move from North Korea. Vice Minister of Unification Chun Hae-sung told reporters that the North’s decision to pull out is “unfortunate and regrettable.”

“Though North Korean staff has pulled out, South Koreans at the office will continue working,” Chun said. According to a Unification Ministry statement, North Korea notified that it “will not mind the South remaining in the office.”

The liaison office opened last September in the city of Kaesong just north of the border between the two Koreas to foster closer bilateral ties. Kim and Moon reached an agreement to open the office during their historic summit meeting in April.

Rapprochement with the North has been a centerpiece of Moon’s presidency, and it has been in tatters since the dramatic summit breakdown in Hanoi.

A similar liaison office in Washington and Pyongyang had also been discussed as a potential outcome of last month’s summit between Trump and Kim. Hours before their meeting was announced, Kim said he would “welcome” a U.S. liaison office in Pyongyang, while Trump praised it as “a great thing.”

In the weeks following the summit breakdown, the Trump administration on Thursday moved to harden sanctions against North Korea. The U.S. Treasury identified two Chinese shipping companies for evading U.S. and international sanctions on North Korea.

The site of the office also was symbolic. Kaesong was the centerpiece of a now-closed special economic zone created by North Korea that brought in some companies from the South. The Kaesong project offered South Korean manufacturers cheap labor and provided the North with foreign currency. It was suspended in 2016 amid Seoul’s anger over the North’s nuclear tests.

Cheon Seong-whun, a former security adviser to the South Korean president, said the North’s move appears to reflect frustration over expectations that Moon’s government could persuade the Trump administration to lift sanctions.

“The failure to do so has got Pyongyang deeply frustrated over Seoul’s role,” he said.

Meari, a state-run outlet of North Korean propaganda for external audiences, dismissed Seoul’s proposal to mediate nuclear negotiation between Pyongyang and Washington. “South Korea talks loudly about enacting inter-Korean agreements but cannot take any practical actions out of concern about their master Americans.”

Since the breakdown of the Hanoi summit, the U.S. administration has insisted that the diplomatic process remains alive, and that it is keen to resume talks with North Korea to find a way forward.

Nevertheless, there have been inescapable signs of a hardening in the U.S. position, mostly notably in the prominent role that national security adviser John Bolton has played in the media post-Hanoi presenting the U.S. position.

Bolton has repeatedly insisted that sanctions cannot be lifted until North Korea fully surrenders its entire nuclear arsenal, as well as any chemical and biological weapons. At the same time, U.S. envoy Stephen Biegun appears to have backtracked from his pre-Hanoi stance — that both sides can take mutual confidence-building steps together — and is now insisting that denuclearization cannot be achieved “incrementally.”

The North Korean position has similarly stiffened.

Last week, Vice Foreign Minister Choe Son Hui warned that Pyongyang was considering suspending talks with Washington unless the United States changes its stance following the breakdown of the Hanoi summit.

Choe accused the United States of taking a “gangster-like stand” but also appeared to leave a small window open for diplomacy by describing the chemistry between President Trump and North Korean leader Kim Jong Un as “mysteriously wonderful.” She said a final decision on whether to continue talks would be announced soon by Kim.

Satellite images showed North Korea has completed reconstruction of a rocket launchpad and rocket engine testing station at the Sohae Satellite Launching Station, with work beginning even before the Hanoi summit. Activity was also seen at a site in Sanumdong, just outside Pyongyang, which is North Korea’s primary site to develop ballistic missiles and space launch vehicles.

The work led some experts to conclude that the launch of a space rocket was being considered — although no further activity was seen in subsequent days, and it is far from clear that such a launch will actually go ahead.

Vipin Narang, an associate professor of political science at MIT, described the latest development as “ominous” but agreed it was more likely a pressure tactic than a sign of an irrevocable rift.

“The optimistic view is it is very calibrated signaling designed to get the U.S. to move away from insisting on complete surrender up front,” he said.

“The pessimistic reading, which I don’t yet share,” he added, “is that Kim has decided after Hanoi that it’s over and that he’s lost the will to negotiate further, and is now just prepping the battlefield, quite literally, for a return to hostile relations.”

Denyer reported from Naoshima, Japan.