A short-range ballistic missile was launched in the early morning hours, the South's military Joint Chiefs of Staff said in a statement. About 10 minutes later, a second missile, this one as yet unidentified, was detected, it added.

"Our military has strengthened surveillance and vigilance in preparation for further launches," the JCS said, adding it had shared information about the incidents with the United States and Japan.

The North's official Korean Central News Agency (KCNA) did not offer immediate confirmation of the launches.

Last week, North Korea claimed to have successfully tested a multiple warhead missile, but the South said that launch ended in a mid-air explosion.

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 missives 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 resumed propaganda loudspeaker broadcasts and live-fire drills near the border.

South Korea also has 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 leader Kim Jong Un in Pyongyang in June in a show of unity.

On Sunday, Pyongyang condemned joint military drills by South Korea, Japan and the United States, calling them an "Asian version of NATO" and warning of "fatal consequences".

The three-day "Freedom Edge" exercises included preparation in ballistic missile and air defences, anti-submarine warfare and defensive cyber training.

Pyongyang has always decried similar combined exercises as rehearsals for an invasion, but Seoul said Sunday the latest exercises were a continuation of defensive drills held regularly for years.