US National Security Adviser H.R. McMaster has warned against an "illusion of success" on the Korean peninsula, as the White House remains deeply sceptical of Pyongyang's participation in the Pyeongchang Winter Olympics in the South next month.

General McMaster told journalists: "Everyone recognises that we can't… fall for what in the past has been a North Korean ploy to create the illusion of success and talks, and to use that to lock in the status quo as the new normal.

"The danger is growing and it's important for all of us to recognise the North Korean regime for what it is, and how grave a threat this is to the world," he said.

Central Intelligence Agency director Mike Pompeo said the agency believed North Korea's missile programme was aimed at coercion, not just self-defence. Pyongyang's next logical step would be to develop the capability to fire multiple missiles, said Mr Pompeo, speaking at the American Enterprise Institute.

The thaw in relations between the two Koreas came after recent talks at the border village of Panmunjom.

North Korean athletes will march with their South Korean counterparts as one team at the opening ceremony of the Winter Olympics, which takes place from Feb 9 to 25.

The two sides will field a unified women's hockey team.

This will be the fourth time North Korea is joining the games with the South, after 2000, 2004 and 2006.

Despite the developments, some experts believe the impasse with the United States will continue. In the US, there is no appetite for a return to talks, said Dr Michael Green, senior vice-president for Asia at the Centre for Strategic and International Studies (CSIS).

"The persistent objective for the North Koreans is pretty clear: they want to have a dialogue with the United States as a fellow nuclear weapons state… akin to US-Soviet arms control (talks in the 1970s and 1980s)," he told The Straits Times.

"Veterans in both Republican and Democratic administrations... know there's a cycle to this. They will get concessions. They will lower the pressure. They will divide the US and China and South Korea. And then they'll go back to testing."

If the policy community in the US is sceptical, it is not alone. The South Korean public, who have welcomed the thaw in relations with North Korea, are also suspicious.

Opinion polls show a degree of disapproval of North Korea's "hijacking" of the Winter Olympics.

"It's probably better for the world that North Korea's in the Olympics, but it's unlikely to be a game changer in terms of the problem - and will probably be back to a severe situation again in the spring or summer," Dr Green said.

Dr Sue Mi Terry, senior fellow and Korea chair at CSIS, said: "It makes sense for Kim Jong Un (to participate in the Olympics). The South Korean government is 100 per cent funding this, so why not?

"North Korea gets to have a complete image makeover on a world stage. It's an opportunity to create fissures between Seoul and Washington and to see what North Korea can get out of South Korea."

But she warned: "We know historically that the Kim family is not in a habit of giving away things for nothing. They never do anything for free."

In the journal The Hill on Jan 22, Professor Sung Yoon Lee of Korean Studies at the Fletcher School of Law and Diplomacy wrote: "Kim will mesmerise South Koreans, capture the global spotlight, defuse tension and achieve a dramatic self-makeover as peacemaker.

"The new fuzzy atmospherics will allow Kim to buy time and funds with which to perfect his weapons, while painting Trump as the perpetually petulant party."

But a return to testing missiles and nuclear devices after the Olympics carries the very real risk of a US strike, analysts say.

"The only thing stopping that is Defence Secretary James Mattis," a security analyst said. "If he says okay, it will happen."

While Mr Kim has claimed his country's nuclear programme is "complete", experts still doubt that Pyongyang has the ability to deliver a nuclear warhead over thousands of miles with pinpoint accuracy.

That scepticism may prompt him to try and prove it, warned Dr Terry.

She said this year is a likely turning point in the crisis and could see anything from a dramatic lowering of tension to a tip into war.

"Ironically, we have to rely on Kim Jong Un to make rational choices to avoid a conflict."