"North Korea sent unknown ballistic missile towards the East Sea," South Korea's military said, referring to the body of water also known as the Sea of Japan.

Monday's launch took place while outgoing US Secretary of State Antony Blinken was meeting his counterpart and the South's acting president before he heads to Japan.

The missile appeared to have fallen into the sea, the Japanese defence ministry and Japanese coast guard said in separate statements.

The ballistic missile launch is Pyongyang's first this year. Its previous launch was in November, when it test-fired what it said was its most advanced and powerful solid-fuel intercontinental ballistic missile (ICBM).

That was North Korean leader Kim Jong Un's first weapons test since sending soldiers to help Russia fight Ukraine.

South Korea fired a ballistic missile into the sea in a show of force in response to Pyongyang's salvo of missile launches.

North Korea also staged GPS jamming attacks on the South later in November, an operation that affected several ships and dozens of civilian aircraft in the country.

South Korea has been plunged into weeks of political crisis after President Yoon Suk Seol announced a short-lived martial law decree on December 3.

North Korean state media said last week the South was in "chaos" and paralysed politically over an attempt by investigators to execute an arrest warrant for Yoon.

- N. Korea-Russia ties -

Monday's test-fire was also the first since President-elect Donald Trump won re-election in November.

North Korea and Russia have strengthened their military ties, to the concern of US allies, since Moscow's invasion of Ukraine in February 2022.

A landmark defence pact signed by Pyongyang and Moscow in June came into force last month, with Russian President Vladimir Putin hailing it as a "breakthrough document".

It obligates both states to provide military assistance "without delay" in the case of an attack on the other and to cooperate internationally to oppose Western sanctions.

North Korea and Russia are under rafts of UN sanctions -- Kim for his nuclear weapons programme and Moscow for the Ukraine war.

The North's deployment of troops has led to a shift in tone from Seoul, which has so far resisted calls to send lethal weapons to Kyiv.

However, the South's government indicated before its political crisis began that it might change its longstanding policy.