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South Korea has called on North Korea to explain why shots were fired across its heavily fortified border on Sunday.
Seoul said bullets fired from the North hit a frontline guard post before South Korean troops fired 20 warning shots in response.
It was the first exchange of gunfire in the demilitarised zone between the countries in about two years, but there were no known casualties on either side, South Korean defence officials said.
On Monday, South Korean Defence Ministry spokeswoman Choi Hyun-soo said Seoul sent a message of strong protest against the gunfire to Pyongyang.
The South urged its neighbour to explain the incident and to avoid similar acts.
Ms Choi said North Korea had yet to respond.
The 248-kilometre demilitarised zone bisects the Korean Peninsula and is lined by mines, barbed-wire fences and troops on both sides.
It was formed as a buffer after the end of the Korean War in 1953 and is jointly overseen by North Korea and the US-led UN Command.
On Monday, the command said it would investigate whether there was a breach of an armistice that ended the war.
South Korean military spokesman Kim Joon-rak declined to comment on the investigation.
The exchange of gunfire happened two days after North Korean leader Kim Jong-un appeared in public for the first time in three weeks, an absence that prompted intense rumours about his health.
The incident also came amid deadlocked US diplomatic efforts to rid North Korea of its nuclear weapons.
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On Sunday, South Korea's military said a preliminary analysis showed the gunfire was probably not a calculated provocation.
US Secretary of State Mike Pompeo also said the gunfire was not intentional.
But some observers doubt it was an accident and said North Korea might be planning further provocations to try to secure diplomatic concessions.
On Saturday, North Korean media published images of Mr Kim at a ribbon-cutting ceremony for the inauguration of a fertiliser plant, after reports he was ill.
On Monday, a former senior North Korean diplomat apologised after suggesting Mr Kim was likely so ill he could not stand, days before the North Korean leader attended the event during which he was seen chain smoking and walking briskly. Hundreds of people attended the ceremony.
Mr Kim's re-emergence also came as a blow to the credibility of high-profile defectors from the North who speculated Mr Kim could even be dead.
One of the defectors, Thae Yong-ho, was North Korea's deputy ambassador to Britain, where he managed secret funds for Mr Kim.
Mr Thae fled to South Korea in 2016 and was one of two defectors elected to parliament last month.
"I am aware that one of the reasons why many of you voted for me as a lawmaker is with the expectations of an accurate analysis and projections on North Korean issues," Mr Thae said.
"I feel the blame and heavy responsibility. Whatever the reasons, I apologise to everyone."