SEOUL—North Korean soldiers laid three land mines that exploded a week ago on the South Korean side of the nations’ heavily fortified border, the United Nations and Seoul’s military said. The explosions severed the legs of two South Korean soldiers.

In response, South Korea resumed anti-North Korean propaganda broadcasts through large loudspeakers at the border Monday evening, after more than a decade off. Relations between the Koreas are souring ahead of large military drills in South Korea that begin next week, annual exercises with the U.S. that typically draw an angry reaction from Pyongyang.

The mines exploded on the morning of Aug. 4 as eight South Korean soldiers were conducting a routine patrol along the border near the city of Paju. An initial blast of two mines severed both legs of one soldier. Another soldier lost one of his legs in a separate explosion as he helped the first soldier, South Korea’s military said.

South Korean and U.S. officials say North Korea has refused virtually all efforts to establish dialogue in recent months. Some observers say North Korean leader Kim Jong Un’s frequent appearances at military events is a sign of his lack of interest in diplomacy.

South Korean military officials didn’t address how they would attempt to prevent a recurrence of the attack or how the mines were placed without being detected.