A K9 self-propelled artillery shoots during a artillery drill in Yeonpyeong Island on Jan. 5 in this photo provided by South Korea's Ministry of National Defense. Yonhap

North Korea threatened Sunday to take action against South Korea over what it claims is violations of the de facto western maritime border, ratcheting up tensions ahead of a trilateral summit in Seoul.

North Korea's supreme military leadership instructed its army on Friday to take offensive action against the encroachment upon its sovereignty, North Korea's Vice Defense Minister Kim Kang-il said, according to the country's official Korean Central News Agency (KCNA).

"We officially warn that we can never tolerate such continued encroachment on our maritime sovereignty and that we may exercise our self-defensive power on or under the water at any moment," Kim said in a statement carried by the KCNA.

The statement came hours before South Korean President Yoon Suk Yeol will have back-to-back bilateral talks with Chinese Premier Li Qiang and Japanese Prime Minister Fumio Kishida at the presidential office.

The three leaders will hold a trilateral summit on Monday. It marks the first three-way summit among the Asian countries since December 2019, after a hiatus due to the COVID-19 pandemic and strained Seoul-Tokyo relations over historical disputes.

Tension remains high along the Northern Limit Line (NLL) in the Yellow Sea, the de facto maritime border. North Korea has never recognized the NLL, demanding that it be re-drawn further south.

Waters near the NLL have been a flashpoint between the two Koreas, where three bloody naval skirmishes took place in 1999, 2002 and 2009.

In March 2010, Pyongyang torpedoed a South Korean warship near the sea border, killing 46 sailors on board. In November that year, the North bombarded the South's border island of Yeonpyeong, killing two civilians and two Marines.

North Korea also accused South Korea and the United States of intensifying their hostile aerial espionage as it vowed to scatter "mounds of wastepaper and filth" over the inter-Korean border areas in retaliation against anti-Pyongyang leaflets.

For years, North Korean defectors in South Korea and conservative activists have sent balloons carrying leaflets critical of the North's leader Kim Jong-un into North Korea. Pyongyang has bristled at the propaganda campaign amid concern that an influx of outside information could pose a threat to the Kim regime.

North Korea blew up the joint liaison office in the now-shuttered inter-Korean factory park in the North Korean border city of Kaesong in 2020 in anger over Seoul's failure to stop North Korean defectors from sending anti-Pyongyang leaflets across the border. (Yonhap)