South Korea and the United States, which have ramped up defence cooperation in response to growing threats from the nuclear-armed North, are currently carrying out joint large-scale live-fire "annihilation" exercises.

Seoul's Joint Chiefs of Staff said it had detected the launch of "two short-range ballistic missiles from the Sunan area into the East Sea between 19:25 and 19:37 (1025 to 1037 GMT)," referring to the body of water also known as the Sea of Japan.

"We have stepped up monitoring in case of further provocations and are maintaining readiness in close coordination with the United States," it said, adding that the launches were a "grave provocation" which violated UN sanctions.

Tokyo also confirmed the missile launches, with a defence ministry official telling reporters that the two missiles had landed in waters within Japan's exclusive economic zone.

"The missiles may have flown on irregular trajectories," Japan's top government spokesman Hirokazu Matsuno told reporters, adding that one had flown 850 kilometres (530 miles) and the other around 900 kilometres at altitudes of 50 kilometres, before landing in Japan's EEZ.

Relations between the two Koreas are at one of their lowest points in years, with diplomacy stalled and the North's leader Kim Jong Un declaring his country an "irreversible" nuclear power, as well as calling for ramped-up weapons production, including of 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 and Japan, including expanding joint drills, which had been scaled back because of Covid-19, and during a bout of ill-fated diplomacy.

Yoon personally watched South Korean and US troops take part in the live-fire exercises Thursday.

All such drills infuriate Pyongyang, which regards them as rehearsals for invasion.

North Korea released a statement Thursday slamming the drills, with a defence ministry spokesperson saying they were "targeting the DPRK by massively mobilizing various types of offensive weapons and equipment", referring to the country by its official name.

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

It added that the drills were "escalating the military tension in the region", and warned: "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 for 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 (for) the North's demolition of the Kaesong office," Choi Gil-il, professor of military studies at Sangji University, told AFP.