SEOUL, South Korea — After a period of calm — or relative calm, at least — along the heavily militarized border between North and South Korea, both sides were back on alert Friday. The unlikely cause: Loudspeakers.

North Korea said Friday morning that its leader, Kim Jong-un, had ordered military units to be ready to attack loudspeakers near the border that the South has used in recent days to blare propaganda messages. Mr. Kim gave the South until 5 p.m. Saturday to stop using the speakers. If not, the North promised “strong military action,” though it did not say when it would act.

Bellicose demands, threats and ultimatums from the North are hardly uncommon, and each side tends to react angrily to any move from the other that it sees as provocative. Frictions sometimes escalate all the way to exchanges of gunfire, only to ease back from the brink again.

Still, the latest spike in tensions came the day after the North directed artillery fire and what may have been a rocket across the border, according to South Korea, prompting a response in kind from the South. It was the first exchange of fire across the border on such a scale in five years.