North Korean soldiers briefly crossed the demilitarised zone (DMZ) on Tuesday, South Korea has said, ahead of a rare state visit to Pyongyang by Russian President Vladimir Putin.
The soldiers retreated after the South Korean military fired warning shots, but Seoul believes their violation of the border was not intentional.
South Korea said it believed the soldiers accidentally crossed as they were fortifying the border, the second such incident in over a week.
The border itself is not fenced and the signposts are obscured by heavy growth of vegetation.
"20 to 30 soldiers came about 20 meters below the military demarcation line (MDL) inside the demilitarized zone (DMZ)," South Korea's Joint Chiefs of Staff (JCS) said in a statement.
It's the second crossing incident in two weeks, with Seoul saying North Korean troops crossed into South Korea on 9 June with tools, including pickaxes. They retreated when South Korean reacted with loudspeaker warnings and warning shots.
The JCS noted that North Korea has been sending a large number of troops into the DMZ to clear vegetation and build fortifications since April.
Military officials in Seoul noted that North Korean troops have suffered multiple casualties while working around the DMZ due to landmine explosions during the construction efforts.
They added that North Korea completed restoring old guard posts in the area in January, before planting new landmines on roads between the countries.
"This work appears to be aimed at strengthening control over North Korean troops and residents, which includes prevention of defection," the JCS said.
The DMZ is one of the world's most heavily fortified areas, a no-man's land that stands as a remnant of the Cold War. From Gyeonggi-do in the west to Gangwon-do in the east, the 160-mile (258km) long DMZ divides the Korean Peninsula in two.
While 196 people fled North Korea to South Korea in 2023, almost none cross the fortified border. Most instead cross into China and then to South Korea.
The latest crossing comes as North Korea prepares to receive Russian President Vladimir for his first visit to the country in more than 20 years, after North Korean leader Kim Jong Un travelled to Russia last year.
The US has expressed concern about the deepening relationship between the two countries, saying it was troubled by the supply of North Korean missiles for the war in Ukraine.
Alongside the US, South Korea has also accused North Korea of supplying Russia with artillery and other equipment, likely in exchange for food and military aid.
There's been heightened tension between Seoul and Pyongyang in the last few weeks, as South Korea resumed propaganda broadcasts towards the North in response to Mr Kim's regime sending hundreds of balloons carrying rubbish across the border.
