US State Secretary Rex Tillerson has said Washington is not trying to topple Kim Jong-un's regime in North Korea, but Pyongyang must halt its nuclear missile programme.

"We do not seek a regime change. We do not seek the collapse of the regime. We do not seek an accelerated reunification of the peninsula," he said during a press conference in Washington, DC, on Tuesday.

"We do not seek an excuse to send our military north of the 38th parallel. And we're trying to convey that to the North Koreans. We are not your enemy. We're not your threat, but you are presenting an unacceptable threat to us, and we have to respond."

Reviewing his six months in office, Tillerson said the US would be willing to talk to the North Korea if it accepts to disarm.

"We don't think having a dialogue where the North Koreans come to the table assuming they're going to maintain their nuclear weapons is productive," he said.

But he sought to reassure the isolated regime that it does not need a nuclear arsenal to defend itself from a US attack.

China's role

US President Donald Trump has demanded that China, North Korea's neighbour and biggest trade partner, rein in its nuclear ambitions - angrily tweeting over the weekend that Beijing is not doing enough.

But here too, Tillerson was more diplomatic.

"We certainly don't blame the Chinese for the situation in North Korea," he said.

"Only the North Koreans are to blame for this situation, but we do believe China has a special and unique relationship, because of this significant economic activity, to influence the North Korean regime in ways that no one else can."

Tillerson's views on North Korea and its close ally, China, is in sharp contrast to the US envoy to the UN, Nikki Haley, who on Sunday asked Beijing to decide on tougher UN sanctions on North Korea over its two long-range missile tests last month.

"China must decide whether it is finally willing to take this vital step. The time for talk is over," she said.

Last week, Kim boasted that North Korea could attack any target in the US after carrying out its latest intercontinental ballistic missile (ICBM) test.

Republican Senator Lindsey Graham said Trump was ready to launch a devastating military attack if diplomacy failed to stop the nuclear missile threat.

Trump has also spoken to its ally Japan over the latest North Korean missile test.