The North Korean military has breached the demilitarized zone (DMZ) separating the communist nation from South Korea several times this month.
North Korean soldiers were seen inside the demilitarized zone on Tuesday morning, apparently working on a project in the prohibited area.
South Korean military officials released photos of the raid by North Korean troops around 8:30 a.m., who the Joint Chiefs of Staff said were carrying work tools.
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South Korean soldiers fired warning shots to scare the North Koreans, who dispersed and re-entered their own borders.
Approximately 20 to 30 North Koreans are believed to have entered the demilitarized zone during the incident.
A similar incursion into the DMZ was reported earlier this month, when a large group of North Korean soldiers crossed the prohibited zone on June 9.
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The June 9 incident sparked a similar protest by South Korean guards, who broadcast warning messages and fired warning shots until the intruders fled the area.
These interactions have raised tensions between the two Koreas just as North Korean leader Kim Jong Un is expected to host Russian President Vladimir Putin this week.
Putin will visit Pyongyang for two days starting Tuesday as he makes his first official state visit to the communist nation in 24 years.
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Russia has kept the much smaller and economically fragile North Korea as an ally while Western powers unleashed heavy sanctions over the invasion of Ukraine.
In recent years, North Korea has served as a valuable intermediary for Russia and China by conducting trade and manufacturing operations outside the scope of international law.