Discussions have grown contentious after a historic summit in June between President Donald Trump and Pyongyang strongman Kim Jong Un, where the leaders pledged to work toward the "complete denuclearization of the Korean Peninsula."

But despite the bonhomie of the occasion, Pyongyang has taken few concrete steps toward that stated goal and signs of growing frustration abound.

Last week, Trump nixed Secretary of State Mike Pompeo's planned trip to North Korea, reportedly because he received what US officials deemed to be a "belligerent" letter from Kim Yong Chol, vice chairman of North Korea's ruling party.

And on Tuesday, Defense Secretary Jim Mattis said the Pentagon is not planning to suspend any more military drills on the Korean peninsula.

"We took the step to suspend several of the largest military exercises as a good faith measure," Defense Secretary Jim Mattis told reporters.

"We have no plans to suspend any more."

Following his summit with Kim, Trump scrapped this summer's massive Ulchi Freedom Guardian joint exercise with South Korea.

Carefully calibrating his words, Mattis would not say when any exercises might resume, apparently leaving wiggle room for North Korea.

"We are going to see how the negotiations go, and then we will calculate the future, how we go forward," Mattis told Pentagon reporters.

The next large-scale US-South Korean drills, known as Key Resolve/Foal Eagle, and are set for next spring.

While North Korea views the exercises as a "provocation," they are loathed by China too. Trump last week berated Beijing, saying they weren't helping enough with the process of denuclearization.

- 'No longer' a threat -

Trump has staked a lot on his talks with North Korea.

He prides himself on being able to make deals and after the summit, he famously declared: "There is no longer a nuclear threat from North Korea."

Vipin Narang, a professor in security studies at Massachusetts Institute of Technology, said recent developments are the first public acknowledgment that Trump is frustrated with the pace of talks.

But he noted North Korea holds vital cards when it comes to setting the tone of the talks.

Pyongyang's moratorium on missile and nuclear testing could be conditional on negotiations continuing in good faith, he said.

"So if all this falls apart we could go back ... Kim Jong Un could do something like a satellite launch," Narang told AFP.

Last month, officials told the Washington Post that Pyongyang seems to be developing at least one or two liquid-fueled intercontinental ballistic missiles.

The work is underway at a factory outside Pyongyang in Sanumdong, where scientists produced the North's first ICBMs capable of reaching the United States.

When asked if the Pentagon agrees with Trump's assertion the North Korea nuclear threat is over, Mattis pointed to the calming of tensions that spiked last year when the US president and Kim traded personal attacks amid apocalyptic rhetoric.

"The whole world saw that progress when the two leaders sat down," he said.

"We also knew very clearly this was going to be a long and challenging effort to negotiate this away as you know that war began in 1950 and has never ended."

Still, several observers say American diplomats believe Kim has no intention of giving up his atomic bombs and is leveraging his relationship with Trump for more concessions.

The North Koreans are "confident they can get what they want from another Kim-Trump summit," noted Daniel Sneider, a lecturer in East Asian Studies at Stanford University.