US Secretary of State Rex Tillerson has said the US is ready to begin talks on North Korea's nuclear disarmament, without prior conditions.

"We're ready to have the first meeting without preconditions," Mr Tillerson told a meeting on the crisis at the Atlantic Council policy forum in Washington.

"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."

This appeared to represent a softening in the US position, seeing as officials had previously demanded that Kim Jong-un's regime demonstrate a willingness to consider giving up its arsenal.

But elsewhere in his remarks, Mr Tillerson insisted the US-led pressure campaign of economic and diplomatic sanctions would continue until "the first bomb drops".

He stressed that the US "simply cannot accept a nuclear armed North Korea" and that President Donald Trump "intends to ensure that they do not have deliverable nuclear weapons to (reach) the shores of the United States".

He also warned that the US military stands ready to act if necessary.

Meanwhile, Mr Kim has vowed to make North Korea the "world's strongest nuclear power," according to state media.

Mr Kim told workers behind the recent test of a new missile that his country "will victoriously advance and leap as the strongest nuclear power and military power in the world."

Tension flared anew in the flashpoint peninsula after the  29 November launch of the Hwasong-15 ICBM, which the North claimed could deliver a "super-large heavy warhead" anywhere on the US mainland.