If United States (US) President Donald Trump and his top diplomat are playing “good cop, bad cop” with North Korea, it doesn't appear to be working: entreaties of diplomacy aren't yielding meaningful talks, and military threats aren't scaring Pyongyang into halting its nuclear advance.

Instead, America's mixed messaging may be increasing the risk of miscalculation by the isolated, communist government, which lacks insight into the Trump administration's thinking and could mistake brinkmanship for an overt threat of war.

Although American military action could invite devastating consequences for its South Korean ally, Trump has threatened to use military options and offered sometimes apocalyptic visions of the North unless it ends its nuclear and missile testing.

North Korea has launched intercontinental ballistic missiles that can potentially strike the US mainland and a month ago conducted its largest ever underground nuclear explosion. It has threatened to explode another nuclear bomb above the Pacific.

Amid all the threats, however, some level of US-North Korean diplomacy has survived.

Speaking last weekend in China, which wants Washington to resume a full dialogue with Pyongyang, US Secretary of State Rex Tillerson fuelled speculation of a new diplomatic effort, acknowledging open channels of communications between the two countries.

Hours later, Trump chimed in.

"I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man," Trump tweeted Sunday, once again deploying his pet name for North Korean leader Kim Jong Un. "Save your energy Rex, we'll do what has to be done!"

The jarring tweet fuelled a narrative that Trump was undermining his chief interlocutor with the world.

But officials close to Tillerson insisted that the pair were on the same page, and that Trump was merely sending a message to North Korea that it would have to show up in serious negotiating mood to any diplomatic talks.

Whatever the intent, confusion was the result.

And on Monday, White House spokeswoman Sarah Huckabee Sanders compounded the impression that the White House views diplomacy as a dead end.

The only conversations the US is willing to have with North Korea are about the fate of the three Americans being held there, she said.

Back-channel talks in the spring led to the release of college student Otto Warmbier, who returned home with brain damage and died days later.

"Beyond that there will be no conversations with North Korea at this time," Sanders said.

While the episode represents the latest example of the president appearing to step on his top diplomat's toes, even if advertently, perhaps more concerning for the United States is the lack of results Trump's hardnosed approach or Tillerson's softer tack have produced.

Trump's “bad cop” may have galvanised action by China, the North's traditional diplomatic protector and main trading partner, which has agreed to tough new United Nations sanctions on its ally's primary sources of revenue for weapons development.

The measures include banning imports of North Korean coal, iron ore and textiles, and new limits on crucial oil and petroleum supplies.

But none of the economic pressure has forced Kim's government to even slow down a nuclear acceleration that could soon put the entire US within range.

Tillerson's “good cop” has hardly fared better. The North professes to not even be interested in diplomacy unless the US abandons its “hostile policy.”

During his meetings in Beijing, the Chinese assured Tillerson they would implement the UN sanctions and press their local authorities near North Korea's border to stamp out banned activity, according to a State Department official familiar with the talks.

China wants the North to halt its nuclear and missile tests, however remote the goal, and is eyeing the winter months when there are no US-South Korean military exercises that often exacerbate tensions on the divided Korean Peninsula. The official wasn't authorised to speak publicly on the matter and demanded anonymity.

Even that appears a remote possibility.

Analysts and even governments in between say Trump is making it increasingly difficult for the North to have confidence in any diplomatic effort with the United States, given his threats to demolish a 2-year-old arms control pact with Iran.

And back-channel diplomacy between senior US and North Korean diplomats primarily aimed at freeing detained Americans hasn't helped repair any trust.

In fact, the two sides have barely communicated for the last couple of weeks and there is no sign North Korea wants to re-engage, according to officials with knowledge of the discussions.

The “good cop, bad cop” ploy is unlikely to work, said Evans Revere, a former State Department official who took part in unofficial talks in Switzerland with North Korean officials last month.

The North, he said, wants only to speak to Washington as “one nuclear power to another. “