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

Kim said the "newly developed" system was a "great weapon," and expressed "high appreciation" for the scientists who had designed and built it, the country's official Korean Central News Agency said.

Kim added the country needed to keep stepping up weapons development "for resolutely frustrating the ever-mounting military threats and pressure offensive of the hostile forces," KCNA noted.

Photos carried by the official Rodong Sinmun newspaper showed a broadly grinning Kim standing in front of an eight-wheeled launcher and rockets shooting out from the large canisters mounted on the vehicle.

"The test-fire proved that all the tactical and technological specifications of the system correctly reached the preset indexes," the report said.

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 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.