"Possible progress being made in talks with North Korea," the US commander-in-chief tweeted, in a first cautious response to the overture transmitted through US ally South Korea.

"For the first time in many years, a serious effort is being made by all parties concerned," Trump tweeted after a South Korean envoy returned from a landmark meeting in the North with Kim Jong Un.

"The World is watching and waiting! May be false hope, but the U.S. is ready to go hard in either direction!" Trump said, keeping options open.

South Korea's national security advisor Chung Eui-yong announced the potential breakthrough, as well as plans for a landmark summit between North and South Korean leaders in the Demilitarized Zone this April.

In return for a security guarantee -- likely a US promise not to attack or overthrow the Kim regime -- Chung said that the North was willing to suspend and ultimately abandon nuclear and ballistic missile programs.

- 'A big deal' -

The United States believes North Korea is testing, and could soon complete, an intercontinental ballistic missile capable of delivering an advanced nuclear device to the continental United States.

That ominous technological breakthrough would put cities like Los Angeles and even New York in striking distance of a unpredictable and hostile regime, something that is unthinkable to many security officials in Washington.

In response, Trump has mounted a campaign of "maximum pressure" on Pyongyang, introducing biting sanctions and keeping open the option of preemptive strikes.

The offer of talks is a tantalizing one for Washington, offering a possible off ramp for the road to an inevitably bloody war. But is also fraught with risks.

"It's a big deal," Jung Pak, a former CIA analyst told AFP. It is the first time in Kim Jong Un's seven-year-rule that he has countenanced the prospect of giving up weapons, she said.

But while this is a new gambit for Kim, it is not new for the North Korean regime.

On multiple occasions his father Kim Jong Il dangled the prospect of talks and denuclearization as a means of buying time and dividing South Korea from its allies.

"It could be that 'maximum pressure' is working and (Kim) wants to loosen sanctions, there might be domestic drivers, there could be rumblings among the elite," she said.

"It is probably a gambit to try and lure South Korea away from the West. It's also related to creating this international prestige for Kim Jong Un," she added.

"We just have to be very clear eyed that we've been down this road before," said Pak.

The US is certain to look for further evidence that North Korea is serious about mothballing weapons, including possible international inspections of nuclear and missile programs.

Trump will also have to decide whether to go ahead with "Foal Eagle" a major military exercise between South Korea and the United States that is scheduled around the same time as the Korean summit.

According to a senior official at the South's presidential office, Kim said during Monday's meeting he would "understand" if the South goes ahead with the delayed exercises that usually infuriate Pyongyang.