WASHINGTON -- An enigmatic North Korean leader takes a secretive train trip to China to affirm fraternal ties and declare a commitment to denuclearization.

It sounds like Kim Jong Un's visit this week, but his father and predecessor Kim Jong Il made similar declarations on a trip to Beijing, months before he died in 2011. Yet North Korea's nuclear weapons development only speeded up.

U.S. President Donald Trump expressed optimism Wednesday after the younger Kim's meeting with Chinese President Xi Jinping, saying there's "a good chance" that Kim will "do what is right for his people and for humanity." But there are plenty of reasons to be skeptical that the U.S.-North Korean summit slated for May will produce the breakthrough that Washington wants.

After a year of escalating tensions, Trump agreed to talks after South Korean officials relayed that Kim was committed to ridding the Korean Peninsula of nuclear weapons and was willing to halt nuclear and missile tests.

That has tamped down fears of war that elevated as Trump and Kim traded threats and insults and North Korea demonstrated it was close to being able to strike the U.S. with a nuclear-tipped missile.

Kim's meeting with Xi offered some reassurance to Washington that "denuclearization" will be up for negotiation if the first summit between American and North Korean leaders in seven decades of animosity takes place.

But while Trump has elevated expectations of what that sit-down would achieve, North Korea has yet to spell out what it wants in return for abandoning a weapons program that Kim likely views as a guarantee for the survival of his totalitarian regime.

The readout of Kim's remarks to Xi as reported by China's state news agency Xinhua strongly indicates Pyongyang is looking for significant American concessions.

"The issue of denuclearization of the Korean Peninsula can be resolved," Kim was quoted as saying, "if South Korea and the United States respond to our efforts with goodwill, create an atmosphere of peace and stability while taking progressive and synchronous measures for the realization of peace."

To many North Korea watchers, that sounds like old wine in a new bottle.

In May 2011, the elder Kim, who was making what would be his final trip to China, told then-president Hu Jintao that the North was "adhering to the goal of denuclearization."

That came months after North Korea had revealed a uranium enrichment plant that gave it a second path for making fuel for atomic bombs.

Abraham Denmark, a former senior U.S. defence official, said the North's latest offer to "denuclearize" still appears contingent on U.S. creating the right conditions. In the past, Pyongyang demanded that U.S. withdraw troops from the peninsula, end its security alliance with South Korea and the nuclear protection it offers its ally.

"It's possible that Kim Jong Un has a different meaning in mind," said Denmark, now director of the Asia program at the Wilson Center think-tank . "So far it sounds like the same old tune."

Ending six years of international seclusion, Kim was spirited into Beijing by special train under tight security like his father before him. He met with Xi, seeking to repair relations that have been frayed as China has supported tough UN sanctions and slashed trade with its wayward ally in frustration over its refusal to stop its provocative behaviour.

State Department spokeswoman Heather Nauert said Kim's first foreign trip was a "historic step in the right direction" and proof that U.S.-led campaign of "maximum pressure" of economic sanctions was working. Trump said that the pressure would be maintained for now, but offered an optimistic view of how he could achieve peace and denuclearization that eluded past administrations.

"Now there is a good chance that Kim Jong Un will do what is right for his people and for humanity. Look forward to our meeting!"

There's another way of looking at it.

It could be North Korea not the U.S. that is calling the shots. When Kim offered an olive branch to South Korea in the new year, he also warned that the entire U.S. was within range of the North's atomic weapons. With that capability in hand, he may now going on a diplomatic offensive, using it as leverage to win aid and security guarantees rather than with an intent of giving it up.

Trump's own choice as national security adviser John Bolton is famously skeptical of diplomacy with North Korea. Just a month ago, he made the case for a pre-emptive military strike on the North. That raises questions about whether he might advocate for the same should Trump's summit with Kim fail.

Experts at the Center for Strategic and International Studies think-tank said in an analysis that by meeting Xi, Kim may be seeking an insurance policy that "even if summit talks fail with the U.S. that North Korea could still fall back on its relationship with China."

Japanese Prime Minister Shinzo Abe, a close ally of the president, is worried about a less than ideal outcome.

He said Wednesday he's worried that in his talks with Kim, Trump will focus on the intercontinental missiles that can reach the U.S. mainland and not the shorter-range missiles that threaten Japan and may "end up accepting North Korea's possession of nuclear weapons."

Associated Press writer Alan Fram contributed to this report.