The launch, which Yonhap said took place around 4 am, come as relations between the two Koreas are at one of their lowest points ever.

"South Korean and US intelligence authorities were analyzing the launches," Yonhap reported.

The launches come only three days after North Korea fired two ballistic missiles into the sea on its opposite coast toward Japan.

The recent missile launches are the latest in a series of weapons tests by Pyongyang, and come as Seoul and Washington ramp up defence cooperation in the face of soaring tensions with the North.

Diplomacy between Pyongyang and Seoul has stalled and North Korean leader Kim Jong Un has called for ramping up weapons development, including tactical nukes.

In response Seoul and Washington have staged joint military exercises with advanced stealth jets and US strategic assets.

On Thursday, North Korea's defense minister Kang Sun Nam said in a statement that the deployment of a nuclear-capable submarine to South Korea "may fall under the conditions of the use of nuclear weapons specified in the DPRK law on the nuclear force policy," using an acronym for North Korea's official name.

Saturday's incident also comes as a US soldier is believed to be in Pyongyang's custody after breaking away from a tour group visiting the Demilitarized Zone.

The United States has said it is "very concerned" about how Private Second Class Travis King would be treated, and that as of Thursday, Pyongyang had yet to respond to inquiries about the soldier.

King was due to return to the United States to face military discipline after serving jail time in South Korea for assault.