South Korean soldiers fired warning shots at North Korean troops after nearly a dozen crossed the demilitarised zone separating the warring nations.

Seoul’s military said about ten North Korean soldiers violated the demarcation line on Tuesday but returned to the North’s territory, marking a potentially significant escalation in tensions between the two countries.

“Our military conducted a warning broadcast and fired warning shots, and the North Korean soldiers moved north,” Seoul’s joint chiefs of staff said in a statement, adding the incident took place at 5pm (8am GMT).

South Korea is closely monitoring North Korean activities across the heavily fortified border that divides the peninsula, they said.

The 155-mile-long Korean Demilitarised Zone (DMZ) cuts the Korean peninsula roughly in half and is the world’s most heavily armed border.

It is not yet clear why the soldiers crossed the border, but it comes amid Kim Jong-un’s increasing threats of war against Seoul.