"North Korea fired an unidentified ballistic missile into the East Sea," Seoul's Joint Chiefs of Staff said, referring to the body of water also known as the Sea of Japan.

Japan also confirmed the launch, with Tokyo's defence ministry saying Pyongyang had "launched a possible ballistic missile" and the country's coastguard calling on vessels to be vigilant and not approach any fallen objects at sea.

Relations between the two Koreas are at one of their lowest points in years, with diplomacy stalled and Kim Jong Un declaring his country an "irreversible" nuclear power, as well as calling for ramped-up weapons production, including tactical nukes.

North Korea has conducted multiple sanctions-busting launches this year, including test-firing its most powerful intercontinental ballistic missiles, and last month attempting to put a military spy satellite into orbit.

In response, the hawkish administration of South Korean President Yoon Suk Yeol has bolstered defence cooperation with the United States, staging regular large-scale joint military drills, including live-fire "annihilation" exercises which are currently ongoing.

Such exercises infuriate Pyongyang, which regards them as rehearsals for invasion.

The ongoing drills were "targeting the DPRK by massively mobilizing various types of offensive weapons and equipment", a spokesperson for the North's Ministry of National Defence said in a statement Thursday, referring to the country by its official name.

"Our response to this is inevitable," they added in the statement, which was carried by the official Korean Central News Agency.

"Our armed forces will fully counter any form of demonstrative moves and provocation of the enemies."

- Lawsuit -

On Wednesday, South Korea filed a lawsuit seeking damages from North Korea for the 2020 demolition of a liaison office.

The office was established in 2018 with funding from Seoul at an industrial zone near the border in North Korean territory, as South Korea's then president Moon Jae-in pressed for a diplomatic breakthrough with Pyongyang.

But after that process collapsed and relations deteriorated, North Korea demolished the building in June 2020.

Seoul said it was seeking 44.7 billion won ($35 million) in damages, with the country's Unification Ministry describing the demolition as "clearly an illegal act".

North Korea is likely to ignore any ruling by the court, but there is precedent in South Korea and the United States of damages being awarded against its government.

"Given the timing, the launch seems like the North's expression of discontent or protest at Seoul's legal action seeking compensation on the North's demolition of the Kaesong office," Choi Gil-il, professor of military studies at Sangji University, told AFP.