In this file photo taken on June 12, 2018, U.S. President Donald Trump, right, meets with North Korea's leader Kim Jong-un at the start of their U.S.-North Korea summit at the Capella Hotel on Sentosa Island in Singapore. AFP

Pyongyang squandered an opportunity to fundamentally reinvent its relationship with the United States during Donald Trump's presidency, Washington's top North Korea envoy said on Thursday, adding he will urge his successors to continue engagement.

Speaking to a think tank in Seoul during a visit to meet with South Korean security officials, U.S. Deputy Secretary of State Stephen Biegun admitted he was disappointed denuclearisation negotiations had stalled and that more progress was not made during his time leading those efforts.

"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," he said, according to his prepared remarks.

Still, he defended Trump's decision to focus on top-level diplomacy with leader Kim Jong Un, and to eschew small steps in favour of seeking a major agreement under which North Korea would surrender its nuclear weapons and the two sides would normalize relations.

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