SEOUL, South Korea — The North Korean leader Kim Jong-un ordered his front-line military units to go on a “semi-war state” on Friday after South Korea said the two Koreas had exchanged rocket and artillery fire in the first major armed clash across their border in five years.

No casualties were immediately reported from the exchange of fire that took place across one of the world’s most heavily armed borders on Thursday. But tensions remained high on Friday, as Mr. Kim ordered his front-line units to be prepared to attack South Korean loudspeakers along the border unless they stopped blaring propaganda broadcasts by Saturday evening.

South Korea, currently in the middle of large-scale joint military exercises with the United States, rejected the North Korean demand. Instead it stepped up its own military vigilance, vowing to retaliate against any provocations from the North.

Mr. Kim ordered all front-line units to go on a “semi-war state,” one of his country’s highest military alerts, from 5 p.m. Friday during an emergency meeting of his Central Military Commission, the north’s official Korean Central News Agency said on Friday.