North Korean soldiers working in the Demilitarized Zone separating the two Koreas briefly crossed into the South again Tuesday before returning to their side after the South fired warning shots, the Joint Chiefs of Staff (JCS) said.

It marked the North's second border violation in less than two weeks after a similar incident on June 9.

Around 20 to 30 North Korean soldiers carrying work tools crossed the Military Demarcation Line (MDL) within the DMZ in the central section of the border at around 8:30 a.m., according to the JCS.

The South Korean military conducted warning broadcasts and fired warning shots, prompting the North Korean soldiers to return to their side of the border, the JCS said, adding that there was no unusual activity after the warning shots.

A JCS official said the incident appears to be another accidental incursion following the June 9 incident.

Earlier this month, some 20 to 30 North Korean soldiers briefly crossed the border at around 12:30 p.m. before retreating back to the North after the South fired warning shots. Four other North Korean soldiers shortly made another incursion but soon returned to the North after similar measures.

"Our military is thoroughly monitoring the North's activities in the border area amid close cooperation with the United Nations Command," the official said.

The border violations came as North Korea has been deploying large numbers of troops in front-line areas since April to conduct an array of activities, such as planting mines, erecting walls and building roads for military operations.

Photos provided by the JCS on Tuesday showed North Korean soldiers carrying out such activities, including building walls, presumed to be barricades for tanks, measuring 4 to 5 meters in height and up to a few hundred meters in length in four different locations.

A JCS official said the North appears to be deploying up to hundreds of soldiers in each site across 10 different locations scattered within the DMZ.

The North appears to be pushing ahead with the border activities despite multiple casualties caused by mine explosions in the area, the official added, without providing further details on the number of affected troops.

"These activities appear to be measures to strengthen internal control of the North Korean military and residents and fend off their attempts to cross the border," the JCS official said.

Inter-Korean ties have sharply deteriorated after North Korean leader Kim Jong-un called for scrapping a decadeslong policy of seeking unification with South Korea and defining their relations as those between "two states hostile to each other" in a year-end meeting.

In January, Kim gave instructions for "strict" measures to block all the channels of inter-Korean communication along the border, such as cutting off the Gyeongui land route, which includes a railway, on the North's side to an "irretrievable level."

The North has since dismantled street lamps and installed mines along its side of two rare roads connecting the two Koreas in an apparent bid to completely shut down the routes once regarded as symbols of inter-Korean cooperation and exchange. (Yonhap)