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.