Dozens of North Korean soldiers briefly crossed the heavily fortified border Tuesday but retreated after warning shots were fired, Seoul said, the second such incident in two weeks as Pyongyang reinforces its frontiers with the South.

Landmine explosions near the border also injured multiple North Korean soldiers, the Joint Chiefs of Staff said, adding that Pyongyang had recently deployed troops in the area to clear scrub and lay mines, as relations between the two Koreas plummet.

The countries technically remain at war as the 1950-1953 conflict ended in an armistice, and the Demilitarized Zone dividing the peninsula is already one of the most heavily mined places on earth.

But North Korea is moving to reinforce that, laying more landmines, reinforcing tactical roads and adding what appear to be anti-tank barriers, Seoul's military said.

The JCS said it believed the Tuesday crossing -- like a previous one on 9 June -- was accidental, with some 20 to 30 North Korean soldiers carrying work tools involved in the incident, which took place around 8:30 am Tuesday (2330 GMT). 

"Dozens of North Korean troops crossed the Military Demarcation Line today... (and) retreated northwards after warning shots" were fired, a JCS official said.

North Korean soldiers tasked with reinforcing the border had suffered "multiple casualties from repeated landmine explosion incidents" but they "appear to be recklessly pressing ahead with the operations," the official said.

"North Korea's activities seem to be a measure to strengthen internal control, such as blocking North Korean troops and North Koreans from defecting to the South," the JCS official said.

Russian President Vladimir Putin is set to visit Pyongyang later Tuesday, sparking concern in Seoul, which has accused the North of shipping arms to Moscow for use in Ukraine in exchange for help with its nascent satellite program.

'No reconciliation'

"This has rather symbolic significance," Koh Yu-hwan, North Korean studies emeritus professor at Dongguk University told AFP, saying adding new mines was making it clear Pyongyang did not want dialogue with the South. 

"By laying mines, North Korea is demonstrating once again that, as per the instructions of the supreme leader (Kim Jong Un), there will be no reconciliation with the South," he added.

"North Korea is not laying mines across the entire frontline, but rather in areas that are easily observable by the South. They are also blocking roads and railways that were previously areas of inter-Korean cooperation."

During a period of warmer ties in 2018, the two Koreas removed landmines along a section of the heavily fortified border in a bid to ease military tensions. 

Earlier this month, around 20 North Korean soldiers crossed the military demarcation line between the two countries in a section of the border "overgrown with trees", according to Seoul's military, which assessed the incursion to be accidental.

That crossing came as North Korea was sending more than a thousand balloons laden with trash southward -- a response, it said, to balloons carrying anti-Pyongyang propaganda sent north by activists.

The South Korean government in turn suspended a 2018 tension-reducing military deal and restarted loudspeaker propaganda broadcasts along the border, infuriating the North, which warned Seoul was creating "a new crisis".

Ahn Chan-il, a defector-turned-researcher who runs the World Institute for North Korea Studies, told AFP that the North Korean military was trying to survey the border area to install more barriers.

"Engineering and observation units have increased their presence in the area. It is believed that the disorderly actions of those who are unfamiliar with the minefields have led to these mine-related accidents."