Kim Jong-un leaves North Korea for Vietnam by train

--First N Korean state media confirmation of 2nd Trump-Kim summit

--Senior officials from first summit, sister with Kim

--Pressure for more specific agreement than first summit

North Korea on Sunday confirmed for the first time that its leader Kim Jong Un will hold a second summit with US President Donald Trump, days ahead of the high-stakes nuclear meeting set to take place in Vietnam's capital of Hanoi.

Kim left Pyongyang by train on Saturday afternoon for the Feb 27-28 summit accompanied by senior North Korean officials, North Korea's official KCNA news agency said.

The delegation with Kim includes top North Korean officials who took part in last year's expanded summit talks between Kim and Trump in Singapore, including top envoy to the US Kim Yong Chol, vice chairman of the central committee of North Korea's Workers' Party Ri Su Yong, and foreign minister Ri Yong Ho.

The North Korean leader's sister Kim Yo Jong, who was also seen aiding him in Singapore, is travelling with Kim as well.

Other senior North Korean officials, such as his de facto chief of staff Kim Chang Son and Kim Hyok Chol, negotiations counterpart to US envoy Stephen Biegun, were already in Hanoi to prepare for the summit.

Kim will also make an official goodwill visit to Vietnam "soon," KCNA said.

On Saturday, Vietnam's foreign ministry announced that Kim would make an official visit to Vietnam in the "coming days" at the invitation of Nguyen Phu Trong, the president and general secretary of its ruling Communist Party.

Trump and Kim will meet in Hanoi eight months after their historic summit in Singapore, the first between a sitting US president and a North Korean leader, where they pledged to work toward the complete denuclearisation of the Korean peninsula.

But scant progress since then has driven the two leaders to again seek a top-down approach to diplomacy, analysts said.

"They will not make an agreement which breaks up the current flow of diplomacy. (President Trump) has mentioned that they'll meet again; even if there is a low-level agreement, they will seek to keep things moving," said Shin Beom-chul, a senior fellow at the Asan Institute for Policy Studies.

The Trump administration has pressed North Korea to give up its nuclear weapons programme, which threatens the United States, before it can expect any concessions.

North Korea wants an easing of punishing US-led sanctions, security guarantees and a formal end of the 1950-1953 Korean War, which ended in a truce, not a treaty.

Both sides are under pressure to forge more specific agreements than were reached at the Singapore summit, which critics, especially in the United States, said lacked detail.

If Kim travels all the way to Vietnam by train, he could take at least 2-1/2 days to make the journey of thousands of kilometres through China.

Beijing has given no details of Kim's trip through China. On Sunday, its foreign ministry did not immediately respond to a request for comment on whether Kim would meet President Xi Jinping before arriving in Hanoi.

On Feb 26, Vietnam will ban traffic on the road Kim is expected to take to Hanoi from a station on the Chinese border, state media said.

Travel by train has been a favourite mode of transport for Kim Jong Un, as it was for his father, Kim Jong Il, and grandfather, Kim Il Sung.

When Kim Il Sung visited Hanoi in 1958, he travelled from Pyongyang to Beijing and then southern Guangzhou by train, finally flying to Hanoi in an aircraft provided by China, its state-run Global Times said on Sunday.