SEOUL, June 17 (UPI) — North Korean soldiers briefly crossed the border on Tuesday morning and were sent back by warning shots, the South’s Joint Chiefs of Staff said, adding that landmine explosions have caused numerous casualties to Northern troops working along frontline areas.

Some 20-30 North Korean troops crossed the military demarcation line, or MDL, in the center section of the DMZ at 8:30 a.m., the JCS said in a message to reporters.

The South Korean military broadcast warning messages and fired warning shots, prompting the North Korean soldiers to return to their side of the border, the JCS said.

A JCS official told reporters Tuesday that the crossing appeared to be accidental, as the North has been ramping up activity in frontline areas of the DMZ since withdrawing from an inter-Korean military agreement in November.

“Various types of work are being carried out as part of strengthening security forces, such as clearing land, laying mines, reinforcing tactical roads and installing unknown structures that appear to be anti-tank barriers,” the official said in a background briefing.

“These activities by North Korea are aimed at preventing North Korean troops and residents from going to South Korea and defecting,” the official said. “This appears to be a measure to strengthen internal control.”

The work in the frontline areas has also led to North Korean soldiers being maimed or killed by landmine explosions in the heavily fortified border area, the official added.

“The North Korean military appears to be carrying out the work excessively despite the fact that several landmine explosions have resulted in numerous casualties,” the JCS official said, without specifying when the explosions occurred.

Tuesday’s incursion was the second in recent weeks by North Korean soldiers. On June 9, South Korea’s military fired warning shots after about 20 North Korean troops briefly crossed the border.

The border incident comes amid rising tensions near the DMZ, with North Korea sending trash-filled balloons across the border earlier this month and the South resuming anti-Pyongyang loudspeaker broadcasts.

Meanwhile, North Korea has been strengthening military and economic ties with Russia in a deal that includes shipping munitions to Moscow for its war in Ukraine.

On Tuesday, Russian President Vladimir Putin was scheduled to meet with North Korean leader Kim Jong Un on his first visit to the country since 2000.