North Korean leader Kim Jong Un crossed into Vietnam Tuesday after a marathon train journey for a second summit showdown with Donald Trump, with the world looking for concrete progress over the North's nuclear programme.

Following a historic first meeting in Singapore in June that produced a vaguely worded statement about Kim's nuclear arsenal, analysts say the second summit in Vietnam's capital Hanoi must this time put substance ahead of bonhomie.

Kim's trademark olive green train chugged into the Vietnamese station of Dong Dang following a 4,000-kilometre (2,500-mile), two-and-a-half-day odyssey shrouded in secrecy.

Schoolchildren waving North Korean flags and a military guard of honour in pristine white uniforms greeted Kim, the first North Korean leader to visit Vietnam since his grandfather Kim Il Sung in 1964.

Security was ultra-tight with scores of heavily armed Vietnamese soldiers and police guarding the site.

Authorities have shut down the 170-kilometre stretch of road to Hanoi that Kim is due to take.

Trump was taking a more conventional route to the meeting on Air Force One and was expected to arrive in Hanoi late Tuesday.

He tweeted he was looking forward to a "very productive" second meeting with Kim.

The US president again dangled the carrot of economic progress for North Korea if it gives up its nuclear programme.

"With complete Denuclearisation, North Korea will rapidly become an Economic Powerhouse," tweeted Trump. "Without it, just more of the same."

"Chairman Kim will make a wise decision!" Relations between the two mercurial leaders have undergone a dramatic turnaround, from flinging personal insults and threats of destruction to Trump declaring he had fallen "in love" with Kim after their Singapore get-together.

But many North Korea watchers dismissed the Singapore summit as a piece of political theatre that failed to produce a concrete roadmap to denuclearisation and stressed that the Hanoi meeting must deliver more.

"The window for diplomatic progress with North Korea will not remain open indefinitely. The second summit between President Donald Trump and Chairman Kim Jong Un must emphasise substance over pageantry," said Kelsey Davenport from the Arms Control Association.

Concrete details about the summit have been few and far between but White House spokeswoman Sarah Sanders told reporters aboard Air Force One that the two leaders would have dinner together on Wednesday with close advisors.

Diplomatic progress since Singapore has floundered on the definition of "denuclearisation."

Stephen Biegun, the US Special Representative for North Korea, acknowledged last month that Pyongyang and Washington did not have a

"shared agreement of what denuclearisation entails".

The United States has repeatedly demanded the North give up its nuclear arsenal in a final, fully verifiable way.

But Pyongyang sees denuclearisation more broadly, seeking an end to sanctions and what it sees as US threats -- usually including the American military presence in the South, and sometimes in the wider region.

"The ambiguity and obscurity of the term denuclearisation only exacerbates the scepticism about both the US and North Korean commitments to denuclearisation," wrote Shin Gi-wook, director of the Korea Program at Stanford University.

In the run-up to the summit, Trump appeared to lower US demands for Pyongyang, repeatedly saying there was no rush to rid the North of its arsenal as long as missile and nuclear tests stopped.

"I don't want to rush anybody. I just don't want testing. As long as there's no testing, we're happy," said Trump.

He also hinted that more summits would follow the Hanoi meeting, reducing expectations of a dramatic breakthrough in the Vietnamese capital.

Pyongyang insists it has already taken major steps, by not testing ballistic missiles or nuclear weapons for more than a year, and blowing up the entrances to its atomic test site.

The North also wants increased security guarantees, which could come in the form of a declaration of an end to the 1950-53 Korean War -- that ended with an armistice instead of a full peace treaty -- or opening liaison offices.

"I believe that the possibility is there," South Korea's presidential Blue House spokesman Kim Eui-kyeom told reporters about a formal declaration to end the war.

"There is no way of knowing what kind of declaration it might be, but I believe the US and North Korea may reach an agreement."

Such a declaration would signal the first stage of normalising US-North Korean relations, said Go Myong-hyun of the Asan Institute of Policy Studies, and would be an ideal "politically symbolic step" rather than prematurely agreeing to sanctions relief.

Harry Kazianis of the Center for the National Interest said Trump and Kim need to take "at least one step forward on denuclearisation" in Hanoi.

"Nothing would be worse than for either side to come out of the meeting as if it was a waste of time," he said.

AFP