South Korea’s military fired warning shots after North Korean soldiers crossed the Military Demarcation Line near the border on Tuesday, according to the country’s Joint Chiefs of Staff (JCS).

Some 20 to 30 soldiers breached the line by 20 metres (65 feet) inside the demilitarised zone on Tuesday morning and briefly moved back north after warning shots were fired by the South, according to a JCS official.

The JCS does not think the breach was intentional, the Yonhap news agency reported.

North Korean soldiers also suffered multiple casualties while working due to the explosion of landmines in the demilitarised zone, the JCS official told the press.

North Korea’s military has been conducting various activities along the frontline including deploying soldiers and planting landmines, the JCS official added.

Such activities appeared to be part of efforts to tighten border control and prevent North Koreans from defecting to the South, the official said.

“The South Korean military ... is closely tracking the activities of the North Korean military in the frontline area as well as working closely with the United Nations Command,” the official was quoted as saying in a statement.

The incident came as Russian President Vladimir Putin will visit North Korea on Tuesday and Wednesday for the first time in 24 years, according to the two countries.

Last week, South Korea’s military fired warning shots after around 20 North Korean soldiers briefly crossed the border, Seoul officials said.