Pyongyang's state media claimed it had successfully launched the Hwasong-11Da-4.5 missile, which it said was capable of carrying a 4.5 ton-class super-large warhead.

"The test-fire was conducted with a missile tipped with a simulated heavy warhead to verify flight stability and hit accuracy at the maximum range of 500 kilometres and the minimum range of 90 km," the North's Korean Central News Agency (KCNA) said.

Seoul's military had earlier described the Monday launches as being of two short-range ballistic missiles, one of which they said failed early in flight.

The Monday test launches took place after the regime of North Korean leader Kim Jong Un denounced the latest joint military exercises by Seoul, Tokyo and Washington as an "Asian version of NATO" and warned of "fatal consequences".

A super-large warhead, along with a spy military satellite and solid-fuel intercontinental ballistic missiles, was on a list of advanced weapons that Kim vowed to develop at a party congress in 2021.

In the KCNA report, the weapon was "described as a 'super-large warhead', deliberately omitting the word 'nuclear'", Hong Min, a senior analyst at the Korea Institute for National Unification in Seoul.

"But since it is capable of carrying a nuclear warhead, it can be considered a nuclear weapon," he told AFP.

Relations between the two Koreas are at one of their lowest points in years, with the North ramping up weapons testing while bombarding the South with balloons full of trash.

Pyongyang says those missiles are in retaliation for balloons loaded with anti-regime propaganda leaflets sent northwards by activists in the South.

In response to the North's repeated launches, South Korea has fully suspended a tension-reducing military treaty. It also briefly resumed propaganda loudspeaker broadcasts.

On Tuesday, South Korea's military said it had resumed live-fire exercises at artillery ranges near the border with the North.

At front-line ranges within five kilometres of the Military Demarcation Line (MDL) that runs through the middle of the DMZ separating the two Koreas, South Korean troops fired around 140 rounds using the K9 and K105A1 self-propelled howitzers, it added.

Last week, South Korea's Marine Corps also resumed live-fire exercises on islands near the western inter-Korean border, the first such exercises since the 2018 tension-reducing military deal with the North was fully suspended in June.

South Korea has also grown anxious over the North's warming relations with its isolated neighbour Russia.

North Korea is accused of breaching arms control measures by supplying weapons to Russia to use in its war in Ukraine, and Russian President Vladimir Putin held a summit with Kim in Pyongyang last month in a show of unity.