Trump, who had built up expectations for months over his second outing with leader Kim Jong Un, left Hanoi early with no deal -- and no agreed plan for another summit.

North Korea said it offered to close its Yongbyon nuclear plant in return for a partial easing of sanctions, but Trump -- the self-styled master negotiator -- called for complete denuclearization in exchange for full sanctions relief.

"The fact that there was no deal -- and not even a partial deal, despite the fact that President Trump indicated that a small deal was possible -- I think that is a significant disappointment," said Frank Aum, a former top adviser on North Korea to US defense secretaries.

"All that shows to me that everyone came up empty-handed without a clear picture of where we're going in the future," said Aum, a senior expert at the US Institute of Peace.

With Trump already having taken the unorthodox approach of personally negotiating with Kim, observers wondered whether other officials, even Secretary of State Mike Pompeo, could make progress in the face of disagreement between heads of state.

"It's hard to see credible negotiations continuing. The whole point of leader-level talks was to meet with the person who can make actual decisions. That cuts both ways," said Abraham Denmark, director of the Asia program at the Woodrow Wilson Center for International Scholars.

But Aum said that diplomacy still remained viable, seeing as how North Korea made clear it would not resume missile testing and that the United States is keeping its freeze on joint exercises with South Korea -- a goodwill-building dual suspension that had been proposed by China.

"That means there is still hope that over the next couple of years" more progress can be made, Aum said.

- Limited downside to walking away -

The stalemate in Hanoi could paradoxically be a political plus for Trump as members of his Republican Party base had quietly been voicing concern that he sounded desperate for a crowning achievement with North Korea.

"It's better to walk away than sign a bad deal," said Senator Lindsey Graham, a close Trump ally, who also tweeted that it would be time to "end the nuclear threat from North Korea -- one way or the other" if future negotiations fail.

Bruce Klingner, a North Korea expert at the conservative Heritage Foundation, voiced pleasant surprise that Trump "correctly emphasized principles and longtime allies over a premature peace declaration."

Klingner hoped that Trump would stand firm in future negotiations against any quick easing of sanctions or formal declaration of peace with the totalitarian state, a US foe for seven decades.

- No progress, or reversals -

The summit -- which followed Trump and Kim's historic first meeting in June in Singapore -- again appeared to struggle over the meaning of denuclearization, with North Korea seeking a broader end to weapons in the peninsula.

Despite his boasts that he is making progress like no US president before, "Trump's approach has yielded nothing new and has not even come close to resolving the North Korea nuclear threat," said Paul Haenle, a former White House official now at the Carnegie Tsinghua Center think tank in Beijing.

"Once again, Trump returns to Washington without a specific timeline or roadmap for future talks and China's 'suspension for suspension' proposal as the status quo," he said.

But Chris Green, a Korea expert at the International Crisis Group, said that the broad outlines of a deal remained.

"Both parties likely concluded that it would be alright to walk away this time, looking tough for their respective domestic audiences by going home empty-handed," he said.

"Today is a setback, but dialogue seems set to continue. Let's see where we are in another six months."