South Korean President Yoon Suk-yeol. Photo: Xinhua News Agency

Speaking at the celebration of South Korea's West Sea Defense Day, President Yoon described the move as unacceptable.

South Korea will strengthen defense measures to counter North Korea's nuclear and missile activities, and  strengthen security cooperation between South Korea and the United States as well as between South Korea, the United States and Japan, he said.

Earlier Wednesday, the Korean Central News Agency (KCNA) reported that the country had tested a new underwater nuclear attack drone overseen by leader Kim Jong Un.

KCNA also confirmed that Pyongyang launched cruise missiles during weapons tests and drills from Tuesday to Thursday.