Pyongyang fired what appeared to be two short-range ballistic missiles on Saturday, South Korea's military said, the latest in a series of recent launches in protest at the South's joint exercises with the US which wrapped up last week.

North Korea's official news agency described the weapon as a "super-large multiple rocket launcher", and quoted Kim as saying the country needed to keep stepping up weapons development "for resolutely frustrating the ever-mounting military threats".

In France, where he is attending a meeting of G7 leaders, Trump played down the seriousness of the launch.

"I'm not happy about it, but then again he's not in violation of the agreement," Trump said, referring to a deal that prohibits the firing of long-range ballistic missiles.

Trump has staked enormous political capital on personal diplomacy with Kim in a so-far unsuccessful effort to persuade the North Korean dictator to give up his nuclear weapons arsenal.

The latest missile tests could further thwart the resumption of the negotiations.

Photos carried by North Korea's official Rodong Sinmun newspaper Sunday showed a broadly grinning Kim standing in front of an eight-wheeled launcher, and others showing rockets being launched from the vehicle.

Saturday's launch appeared to be "the fourth new missile system that North Korea has debuted since the Hanoi holdup", tweeted Vipin Narang, an associate professor at the Massachusetts Institute of Technology.

"This is Kim's own maximum-pressure campaign," he added.

- Gridlocked talks -

Negotiations between Pyongyang and Washington have been gridlocked since a second summit between Kim and US President Donald Trump in Hanoi in February collapsed without an agreement over the extent of denuclearisation in the North and sanctions relief.

The pair agreed to restart working-level dialogue during an impromptu meeting at the Demilitarised Zone dividing North and South Korea in June, but those talks have yet to begin.

Following Saturday's test, Kim noted August 24 was an "unforgettable good day", recalling successful tests of "strategic submarine underwater ballistic missile" carried out on that day three years ago, KCNA reported.

The comment was "significant" and meant for Trump, said Kim Dong-yub, a researcher at the Institute for Far Eastern Studies in Seoul.

"He is reinforcing that although Pyongyang can't launch them in the dialogue momentum between the US and the North, it possesses these (weapons)," Kim Dong-yub said.

Trump has previously played down Pyongyang's tests and insisted that his personal relationship with Kim remains ideal.

Earlier this month Trump tweeted that Kim had sent him a letter expressing hope that talks would resume once the joint exercises -- which ended on August 20 -- are over.

Washington stations nearly 30,000 troops in the South to help defend its security ally from the nuclear-armed North.

Last week, Stephen Biegun, the US special envoy for North Korea, said during a visit to Seoul that Washington was "prepared to engage" as soon as it hears from Pyongyang.

But on Friday, Pyongyang vowed to "remain as the biggest 'threat' to the US" if Washington persisted with sanctions and said the joint drills between Seoul and Washington had "complicated" prospects for nuclear talks.

In a statement carried by KCNA, the North's foreign minister Ri Yong Ho also launched a scathing attack on Pompeo, calling him a "diehard toxin" and saying it was "sceptical" whether it can negotiate with him.

"We are ready for both dialogue and stand-off," Ri said.