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.
A heavily fortified strip of no-man’s land separating the north from the south, the DMZ border is not fenced and the signposts are obscured by dense vegetation.
About 20-30 soldiers entered the zone by about 20m (65ft), 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 the south on 9 June with tools, including pickaxes. They retreated when South Korea used warning shots and gave alerts over a loudspeaker.
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.
The troops have also suffered multiple casualties while working around the DMZ due to landmine explosions during the construction efforts, military officials in Seoul noted.
They added that work by North Korea to restore old guard posts in the area was completed in January, and new new landmines were then planted on roads between the two countries.
"This work appears to be aimed at strengthening control over North Korean troops and residents, which includes prevention of defection," the JCS said.
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 the south 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 the north of supplying Russia with artillery and other equipment, likely in exchange for food and military aid.
There has 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.
