The U.S. envoy to North Korea has accused Pyongyang of having “squandered” the opportunity to improve ties with President Donald Trump‘s administration, with grand promises of denuclearization and sanctions relief remaining unfulfilled as the president prepares to leave the White House.

Deputy Secretary of State Stephen Biegun, who has served as the special envoy to North Korea since 2018, said Thursday that Pyongyang had chosen “obstacles” over “opportunities” during Trump’s term.

Biegun made the comments at a think tank event in Seoul during a visit to meet with South Korean security officials. Biegun said he was disappointed at the failure to push forward on denuclearization, but said he would encourage President-elect Joe Biden‘s administration to continue engagement with Pyongyang.

“Regrettably, much opportunity has been squandered by our North Korean counterparts over the past two years, who too often have devoted themselves to the search for obstacles to negotiations instead of seizing opportunities for engagement,” Biegun said, according to his prepared remarks reported by Reuters.

Biegun defended Trump’s decision to engage with dictator Kim Jong Un, a strategy that critics warned was handing the leader a priceless propaganda win for little in return. The sudden detente came as a surprise, and followed threats of war from both leaders early in Trump’s term.

Though Trump often lauded his close personal relationship with Kim and celebrated his three historic meetings with the North Korean dictator, the two leaders’ public commitments to denuclearization and sanctions relief came to nothing after working-level discussions stalled repeatedly.

“This vision was a bold one, and it made the many advocates of incrementalism uncomfortable,” Biegun said.

Biden said he will pursue denuclearization of the North, and vowed to take a tougher line on Kim than Trump. During the election campaign, Biden called Kim a “thug” and said the “days of cozying up to dictators are over.”

Biden was part of President Barack Obama‘s administrations that pursued “strategic patience,” i.e. tough sanctions on Pyongyang to try and force its leaders to comply with American demands.

The North has been dismissive of Biden. Last year, Pyongyang branded the president-elect a “rabid dog” that needed to be “beaten to death with a stick.”

Biegun on Thursday urged dialog between the North and the incoming administration. “The war is over; the time for conflict has ended, and the time for peace has arrived,” he said.

But peace appears elusive, as cross border tensions continue despite President Moon Jae-in’s ongoing efforts to build closer cooperation with Pyongyang. This week, a new row has erupted over the COVID-19 coronavirus pandemic, which North Korea claims to have kept out of the country entirely.

South Korean Foreign Minister Kang Kyung-wha told an event in Bahrain last week it was difficult to believe Pyongyang’s assertion that it has no coronavirus cases. Kang also said the North has refused to engage with Southern offers of cross-border antivirus cooperation.

Kim Yo Jong, Kim’s younger sister and a close aide to the young dictator, released a scathing statement on Wednesday warning Seoul would “pay dearly” for maligning Pyongyang’s antivirus campaign.

“It can be seen from the reckless remarks made by her without any consideration of the consequences that she is too eager to further chill the frozen relations between the north and south of Korea,” Kim Yo Jong said of Kang’s remarks.

“We will never forget her words and she might have to pay dearly for it,” Kim Yo Jong said.