South Korean soldiers fired warning shots to repel North Korean soldiers who temporarily crossed the rivals' land border for the second time this month, South Korea's military has said.

About 20 to 30 North Korean soldiers, while engaging in unspecified construction work on the northern side of the border, briefly crossed the Demilitarised Zone dividing North and South Korea at around 8:30am on Tuesday, local time, said South Korea's Joint Chiefs of Staff.

It said the North Korean soldiers retreated after the South fired warning shots and the South's military didn't spot any suspicious activities after that.

The Joint Chiefs of Staff said it doesn't believe the North Korean soldiers intruded the border intentionally and that the North did not return fire.

"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.

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

North Korea's military has been conducting various activities along the frontline including deploying soldiers and planting landmines, the 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 two Koreas remain technically at war as the 1950-1953 conflict ended in an armistice not a peace treaty, with the demilitarised zone and line of control dividing the peninsula one of the most heavily mined places in the world.

Reuters/AP/AFP