North Korea wanted to express to the United States its frustration over the breakdown of the Hanoi summit, when the two sides clashed over sanctions and the extent of Pyongyang's nuclear concessions, analysts say.

South Korea had brokered the now-deadlocked nuclear talks between the US and North Korea, and the White House had described the absence of a missile test for more than a year as a major foreign policy success.

So Seoul, Washington and Pyongyang all have an incentive to play down the nature of the weekend drills.

"President Trump's biggest North Korea policy achievement has been the suspension of Pyongyang's nuclear and missile tests since late 2017," Hong Min, a senior researcher at the state-run Korea Institute for National Unification, told AFP.

"But if we declare the drill involved ballistic missiles at this point, it would all but shatter his self-touted success."

North Korea's official KCNA news agency said the Saturday drills involved "long-range multiple rocket launchers" -- which are not targeted by UN sanctions resolutions -- and unspecified "tactical guided weapons".

But unlike rockets, missiles have guidance systems, and analysts said the images released by state media showed a device similar to Russia's single-stage Iskander missile. It appeared to be one displayed at a North Korean military parade last year, just as the diplomatic rapprochement on the peninsula began.

South Korea's military initially said the North had fired "short-range missiles" but within an hour was describing them as "projectiles".

They had a range of around 70 to 240 kilometres (45 to 150 miles) and fell into the sea, it said, but further analysis was required on whether they were short-range ballistic missiles.

Similarly, US Secretary of State Mike Pompeo avoided calling them missiles, describing them as "short range" in TV interviews, and telling ABC it "didn't present a threat" to the United States, South Korea or Japan.

"We hope that we can get back to the table," he added.

- 'Ratcheting up tension' -

Pyongyang had not launched any missile since November 2017, and last year leader Kim Jong Un declared the development of the North Korean atomic arsenal complete, announcing an end to nuclear and ICBM tests.

While a short-range test would not violate that, any ballistic missile firing could inflict "tremendous damage" on the North-US talks process, said Hong.

And the respected 38 North website said in a commentary that Saturday's drills did not amount to a policy reversal by Pyongyang, which would "require a major and wrenching strategic line change".

"That could still be done but is unlikely to be undertaken only two months after the Hanoi Summit," it added.

But many South Korean conservatives -- who take a more hawkish stance on the North than the liberal Moon Jae-in government -- say Pyongyang did fire ballistic missiles and Seoul's reaction was putting national security at risk.

"The North's missiles pose a threat to lives and safety of South Koreans," said MP Na Kyung-won of the conservative opposition Liberty Korea party.

"But our military and intelligence are only trying to downplay the risk."

The Russian Iskander can carry a small nuclear warhead, the conservative JoongAng Ilbo newspaper added in an editorial Tuesday, and a North Korean version could put "two thirds" of the 20 million people living in or near Seoul at risk.

"Even though North Korea is ratcheting up tension, its time-honoured strategy, our government barely reacts."