Some 20 to 30 soldiers crossed over the line, which runs through the middle of the demilitarised zone (DMZ), by 65 feet before moving back north.
The South Korean military said it believed the North Korean soldiers had strayed across the line by mistake.
Putin is set to visit communist-ruled Vietnam for a two-day trip immediately after visiting North Korea, a move that has angered the United States, Vietnam’s largest trading partner.
Vietnam trip angers US
“No country should give Putin a platform to promote his war of aggression and otherwise allow him to normalise his atrocities,” a spokesperson for the American embassy in Hanoi told Reuters this week.
“If he is able to travel freely, it could normalise Russia’s blatant violations of international law.”
Moscow has long enjoyed deep ties with Vietnam, with thousands of Vietnamese citizens studying in the Soviet Union during the Cold War – including the head of the Communist Party, Nguyen Phu Trong.
Historically, Russia has been Hanoi’s largest weapons supplier – and it continues to extract oil and gas from Vietnamese fields in the disputed South China Sea.