North Korea's decision to meet US President Donald Trump proves the US strategy of isolating North Korea is working, US Vice-President Mike Pence says.

The US has made "zero concessions", he said, and would maintain pressure until denuclearisation was achieved.

The agreement to hold an unprecedented summit stunned observers, reports the BBC.

But the White House later said North Korea would have to take "concrete steps" before any meeting.

Only months ago, Trump and North Korean leader Kim Jong-un were hurling insults at each other.

But now Trump has agreed to the summit following an invitation delivered by South Korea. No sitting US president has ever met a North Korean leader.

President Trump tweeted that a "deal with North Korea is very much in the making". Earlier he has hailed the move as "great progress", but he said sanctions would remain in place until a denuclearisation deal was reached.

The North has halted missile and nuclear tests during previous talks, only to resume them when it lost patience or felt it was not getting what it demanded, analysts say.

Some expressed concern the Trump regime could "fall into the North Korean trap" of granting concessions with nothing tangible in return.

Later, White House spokesperson Sarah Sanders said the president would demand "concrete steps and concrete actions" from North Korea before any meeting took place.

She did not specify what those steps were, nor repeat previous assertions that the meeting would take place by May.

There has been no mention of the developments as yet on North Korean state media.

In Sweden, meanwhile, local diplomatic sources were quoted as saying the North's Foreign Minister Ri Yong-ho would be visiting his Swedish counterpart soon.

The Swedish embassy in Pyongyang represents US, Canadian and Australian diplomatic interests there.

South Korean President Moon Jae-in has acknowledged there are obstacles ahead. He is managing expectations and so much can go wrong.

His approval ratings took a hit during the Winter Olympics after he integrated the women's hockey team with players from the North and met a general from Pyongyang who had been accused of masterminding deadly attacks on South Koreans, though they have since rebounded.

These talks are a huge gamble with a communist state which is hard to read.

But if, just if, he helps pull it off, it may reduce the threat of nuclear war and he could win himself a Nobel Peace Prize.

If all fails, it is back to brinkmanship.