SEOUL, South Korea — South Korea’s military said Monday it concluded North Korea planted mines that injured three South Korean soldiers at their heavily fortified border last month, urging the North to apologize and remove other mines.

The announcement came days after North Korea denied responsibility for the Sept. 21 blasts and threatened “merciless” retaliation if provoked over the case. It responded to South Korea’s earlier, preliminary finding that said there was a “very high possibility” that North Korea-buried mines caused the explosions.

On Monday, South Korea’s Joint Chiefs of Staff said it reached a final conclusion that North Korean mines planted south of the border exploded and injured the three soldiers who were working on creating a patrol route in a densely vegetated area. It cited a joint investigation of the site with the American-led U.N. Command, forensic analysis of TNT and other substances collected from the scene and one other North Korea-planted mine discovered days after the blast.

“Our military strongly urges North Korea to offer a sincere apology to our injured soldiers and immediately remove the mines that it installed in violation of an armistice agreement” that ended fighting in the 1950-53 Korean War, Lt. Gen. Kwon Dae-won, vice chairman of South Korea’s Joint Chiefs of Staff, told a televised briefing.

The armistice agreement prohibits both sides from committing any hostile act “within, from, or against” the Demilitarized Zone, a border buffer zone, and bans military or civilian personnel from crossing the military demarcation line without authorization from the armistice commission.

Despite its name, the DMZ is one of the world’s most heavily armed border areas, with an estimated 2 million mines littered inside and along the zone. North Korea in recent years has added mines, anti-tank barriers and other front-line fortifications along the border.

Kwon said the South Korean military removed a North Korean minefield around the blast site on Monday and will remove other North Korean minefields placed south of the border unless North Korea voluntarily eliminates them.

South Korea’s military earlier said it would take unspecified corresponding steps against North Korea if the North is conclusively found responsible for the mine blasts. But South Korea didn’t immediately announce any substantial punitive steps against North Korea on Monday.

In 2015, mine blasts at the border blamed on North Korea pushed the two Koreas to the brink of an armed conflict, after South Korea’s then conservative government resumed front line propaganda broadcasts for the first time in 11 years.

The latest mine explosions came at a delicate time when South Korea’s liberal government is making efforts to coordinate a resumption of talks between North Korea and the U.S., hoping that would help restart its own talks with the North and boost peace on the Korean Peninsula.

North Korea has been shunning all forms of dialogue with South Korea and the U.S. since its leader Kim Jong Un’s high-stakes nuclear diplomacy with U.S. President Donald Trump fell apart in 2019.