US urges talks as North Korea brags of nuclear arsenal

But even as Washington stressed the importance of a negotiated end to the standoff, Kim Jong-un vowed to make his nation "the strongest nuclear power and military power in the world."

President Donald Trump has promised that Kim will not be allowed to complete his effort to develop nuclear-armed intercontinental ballistic missiles capable of reaching mainland US cities.

To this end, Tillerson has overseen a global diplomatic effort to isolate Pyongyang and stifle its economy through UN sanctions and, ultimately, the threat of US military force.

On Tuesday, in two public appearances, he warned that these efforts would continue until "the first bomb drops" and that Washington "simply cannot accept a nuclear-armed North Korea."

But he said the door to talks with Kim's regime was open, and left the opening wider than he had before, backing away from his former insistence that Pyongyang accept in principle to disarm.

"We're ready to have the first meeting without preconditions," Tillerson told a meeting of the Atlantic Council policy forum.

"Let's just meet and let's talk about the weather if you want and talk about whether it's going to be a square table or a round table if that's what you're excited about.

"But can we at least sit down and see each other face-to-face and then we can begin to lay out a map, a road map of what we might be willing to work towards," he said.

"It's not realistic to say we're only going to talk if you come to the table ready to give up your program. They have too much invested in it."

Some in Washington will see this as a climbdown.

Sanctions expert Anthony Ruggiero, of the hawkish Foundation for Defense of Democracies, said talks should not begin until the US has a way to verify nuclear testing is not continuing.

"The Trump administration should insist that North Korea commit to denuclearization upfront to avoid the failed negotiations of the past," he warned.

- Devastating conflict -

Earlier, in an end-of-year speech to State Department staff and US diplomats, Tillerson stressed the importance of the sanctions regime in forcing North Korea to the table.

Despite hopes that China will do more to cut off oil supplies, international action has forced North Korea's fuel prices up by around a half and cost it $2.3 billion in lost exports, he said.

And Tillerson added that he would see it as a personal failure if his efforts came up short and US Defense Secretary Jim Mattis is forced to take military action to deny the North its arsenal.

But even if Tillerson remains "confident" that the US strategy is paying off, there was little sign of this in Pyongyang, where Kim gave a typically bellicose speech to workers.

According to North Korea's state news agency KCNA, Kim said his country "will victoriously advance and leap as the strongest nuclear power and military power in the world."

His latest boast came after the November 29 launch of the Hwasong-15 ICBM, a missile which the North claimed could deliver a "super-large heavy warhead" anywhere on the US mainland.

Outside experts agree the rocket appears to have the range but some are skeptical that Pyongyang has the technology to allow the rocket to survive re-entry to the Earth's atmosphere.

But US officials have been warning privately for months that Pyongyang is on the verge of a breakthrough, and Tillerson's public confidence appears aimed at much at Trump as at Kim.

Both the US and North Korean leaders have a penchant for warlike rhetoric and personal invective that risks plunging east Asia into a devastating conflict that could leave millions dead.

- 'Fire and fury' -

Trump habitually refers to Kim as "Little Rocket Man" and has threatened to utterly destroy his regime with "fire and fury." North Korea has branded Trump a mentally disturbed "dotard."

In October, after Tillerson revealed he had open diplomatic channels to contact Pyongyang, Trump tweeted that his top diplomat was "wasting his time" trying to talk to Kim.

Further inflaming tensions, in the last week, the US and South Korea launched their biggest-ever joint air exercise, despite calls from China and Russia for a freeze in maneuvers.

Pyongyang slammed the drills as a provocation, accusing Washington of "revealing its intention to mount a surprise nuclear pre-emptive strike" on Kim's administration.