The back-to-back launches follow a warning from Seoul and Washington that any nuclear attack from Pyongyang on the United States and South Korea would result in the end of the North Korean regime.

"North Korea fires unidentified ballistic missile towards the East Sea," Seoul's Joint Chiefs of Staff said Monday morning, referring to the body of water also known as the Sea of Japan.

The Japanese government said on X, formerly known as Twitter, that North Korea fired "what appears to be a ballistic missile(s)", without giving further information.

The launch comes after South Korea said it had detected a short-range ballistic missile fired from the Pyongyang area late Sunday night.

That missile flew around 570 kilometres (354 miles) before landing in the East Sea, the JCS said, adding that Seoul, Washington and Tokyo have "closely shared information regarding North Korea's ballistic missile".

The United States and South Korea on Friday held their second session of the Nuclear Consultative Group in Washington, where they discussed nuclear deterrence in the event of conflict with the North.

A spokesperson for North Korea's defence ministry on Sunday slammed the allies' plans to expand annual joint military exercise next year to include a nuclear operation drill.

"This is an open declaration on nuclear confrontation to make the use of nuclear weapons against the DPRK a fait accompli," said a statement carried by the KCNA news agency, using the official acronym for North Korea.

"Any attempt to use armed forces against the DPRK will face a preemptive and deadly counteraction," it added.

- 'Destabilising impact' -

Sunday's short-range missile launch came as Pyongyang marked the anniversary of the death of leader Kim Jong Un's father and predecessor Kim Jong Il who died on December 17, 2011.

The US Indo-Pacific Command said in a statement that while that launch did not pose a threat to US territory or Washington's allies, it had highlighted "the destabilising impact of the DPRK's illicit weapons programme".

"The US commitment to the defence of the Republic of Korea and Japan remains ironclad," it said.

North Korea last year declared itself an "irreversible" nuclear power and has repeatedly said it will never give up its nuclear programme, which the regime views as essential for its survival.

And last month Pyongyang successfully put a military spy satellite into orbit. It has since claimed its eye in the sky was already providing images of major US and South Korean military sites.