North Korea fired more than 200 artillery shells near two South Korean islands on Friday, Seoul's defence ministry said, warning the actions threatened peace and it would respond.
Residents on the two islands were ordered to evacuate, in one of the most serious military escalations between the two Koreas since the North fired a barrage of shells at one of the islands in 2010.
Friday's live firing followed repeated warnings from Kim Jong Un's regime in Pyongyang that it was prepared for war against South Korea and its US ally.
"The North Korean military conducted over 200 rounds of firing today from around 09:00 to 11:00 (1200 to 0200 GMT) in the areas of Jangsan-got in the northern part of Baengnyeong Island and the northern areas... of Yeonpyeong Island," a defence ministry official said at a briefing.
The ministry released a statement shortly after stating: "This is a provocative act that threatens the peace on the Korean Peninsula."
"We sternly warn that North Korea bears full responsibility for this escalating crisis and strongly urge them to immediately cease these actions," it added.
"Our military closely tracks and monitors the situation in close coordination with the United States, and will take appropriate measures in response to North Korea's provocations."
- Evacuation orders -
Local officials on Yeonpyeong island told AFP that civilians had been told to evacuate, describing the order as a "preventative measure".
South Korea's Yeonpyeong Island is situated in the Yellow Sea. It is located approximately 80 kilometres west of Incheon and 12 kilometres south of the coastline of Hwanghae Province, North Korea.
Authorities on Baengnyeong Island also reported an evacuation order there.
"We are making the evacuation announcements at the moment," a local district official at Baengnyeong island told AFP, adding that he had been told the South Korean military would conduct a naval drill shortly.
"At first I thought it was the shells fired by our own military… but was told later it was by North Korea," Kim Jin-soo, a Baengnyeong island resident told local broadcaster YTN.
Pyongyang fired a barrage of 170 artillery shells onto Yeonpyeong island in November 2010, killing four people including two civilians in the first North Korean attack on a civilian area since the 1950-53 Korean War.
Relations between the two Koreas are at one of their lowest points in decades, after Kim enshrined the country's status as a nuclear power into the constitution while test-firing several advanced ICBMs.
At Pyongyang's key year-end policy meetings, Kim warned of a nuclear attack on the South and called for a build-up of the country's military arsenal ahead of armed conflict that he warned could "break out any time".
Kim accused the United States of posing "various forms of military threat" and ordered his armed forces to maintain the "overwhelming war response capability", according to KCNA's account of the meeting that ended on Saturday.
The meeting concluded that it is a "fait accompli that a war may break out on the Korean peninsula any time due to the enemies' reckless moves for invading the DPRK", KCNA said, using the acronym of the North's official name.
In an effort to deter Pyongyang, Washington deployed a nuclear-powered submarine in the South Korean port city of Busan late last year and flew its long-range bombers in drills with Seoul and Tokyo.
The North has described the deployment of Washington's strategic weapons, such as B-52 bombers, in joint drills on the Korean peninsula as "intentional nuclear war provocative moves".
The military "should rapidly respond to any possible nuclear crisis and put continuous spurs to the preparations for a great event to suppress the whole territory of south Korea by mobilizing all physical means and forces including nuclear forces in contingency", Kim said.
In 2023, the North successfully launched a reconnaissance satellite, after receiving what Seoul claimed was help from Russia in exchange for arms transfers for Ukraine.
China calls for 'restraint'
China called Friday for "restraint" from all sides after Seoul said North Korea had fired an artillery barrage near two South Korean islands.
"Under the current situation, we hope that all relevant parties maintain calm and restraint, refrain from taking actions that aggravate tensions, avoid further escalation of the situation, and create conditions for the resumption of meaningful dialogue," foreign ministry spokesman Wang Wenbin told reporters.
Hours after the reported barrage, South Korean marines on the border island of Yeonpyeong conducted live-fire artillery drills, Yonhap news agency reported.
"We are paying close attention to developments and changes to the situation on the Korean Peninsula," Wang added.
"Confrontations between relevant parties have intensified recently, and the situation on the peninsula continues to be tense.
"As a neighbour of the peninsula, China has always advocated maintaining peace and stability on the peninsula and resolving peninsula issues through dialogue and consultation."
China's close ties with North Korea were on show earlier this week when President Xi Jinping spoke by phone with his counterpart Kim Jong Un on New Year's Day.
The conversation saw the two leaders declare 2024 as a "China-DPRK Friendship Year", according to China's state-run news agency Xinhua, using an acronym for North Korea's official name.
"In recent years, Xi said, the traditional friendly cooperation between China and the DPRK has entered a new historical period with joint efforts," Xinhua reported.
"Under the new situation in the new era, the (Communist Party of China) and the Chinese government have always viewed China-DPRK relations from a strategic and long-term perspective," Xinhua added.
"It is China's unwavering policy to maintain, consolidate and develop the long-standing friendly and cooperative relations between the two countries."