SEOUL, South Korea — An explosion near South Korea’s heavily fortified border with North Korea injured three South Korean military officers on Monday, South Korea’s Defense Ministry said.

The ministry said the explosion occurred during an unspecified operation in the Demilitarized Zone, a border buffer zone separating the two nations that is commonly referred to as the DMZ.

The cause was not immediately clear, the ministry said.

South Korean officials did not say whether it was possible the explosion was caused by a land mine planted by North Korea, which has added mines, anti-tank barriers and other front line fortifications in recent months as relations between the war-divided rivals have sharply deteriorated.

Two of the officers were airlifted from the site by helicopter, while the third was transported by land before being airlifted, according to the ministry. The officers are being treated at a military hospital near the capital Seoul.

The DMZ, which is 248 kilometers (155 miles) long and 4 kilometers (2.5 miles) wide, is one of the world’s most heavily armed borders. An estimated 2 million mines are peppered inside and near the border, which also is guarded by barbed wire fences, tank traps and combat troops on both sides. The zone is a legacy of the Korean War from 1950 to 1953, which ended with an armistice but not a peace treaty.

There have been growing concerns in South Korea about increased activity by North Korean soldiers near the military demarcation line as the North strengthens its border defenses during prolonged tensions between the countries. The South Korean military has not officially confirmed media reports that North Korean soldiers crossed the MDL more than 20 times during front line operations in August alone.

The South’s military fired warning shots to repel some 10 North Korean soldiers who crossed the military demarcation line at the eastern section of the DMZ in April 2025. The North Korean soldiers retreated after the South broadcast audio warnings and fired the shots and there were no clashes. In June 2024, North Korean troops crossed the border three times, prompting South Korea to fire warning shots.

North Korean leader Kim Jong Un in recent years has suspended virtually all cooperation and diplomacy with South Korea while declaring Seoul as his most hostile enemy during a stalemate in his larger nuclear negotiations with Washington.

North Korean mines in past years have occasionally washed down a swollen river into South Korea, killing or injuring civilians. In August 2015, land mine blasts maimed two South Korean soldiers and caused tensions between the two Koreas to flare.