A South Korean soldier standing on guard at the tense inter-Korean border threw a grenade toward a moving object in the dark early on Wednesday, sparking a short-lived alarm, the defence ministry said.
At daylight, a patrol searched the area but there was no trace of any infiltration from North Korea, a ministry spokesman said.
A precautionary alert, which had been issued for South Korean units in the northeastern county of Hwacheon, was consequently lifted.
The episode came amid high tension on the Korean peninsula.
North Korea’s military has put rocket units on a war footing with a fresh threat to strike US targets as well as South Korea. Washington said it was ready to respond to “any contingency”.