South Korea's front-line military unit had been briefly on its highest status of alert after discovering an "unidentified object" in a border fence that separates the two Koreas, but lifted the alert as they found no signs of intrusion from the North, a military official said Wednesday.

A South Korean soldier threw a grenade at the object after discovering it while guarding a military post in Hwacheon, 118 kilometers northeast of Seoul, at around 2:30 a.m. earlier in the day, the official said.

South Korean troops were immediately placed on the highest "Jindogae 1" alert, which is issued to cope with a possible intrusion of armed guerrillas from North Korea, the official said.

After detecting no signs of intrusion from North Korea, the alert was lifted as of 9:20 a.m., the official said.

"The soldier threw a grenade after observing an unidentified object moving and the military unit issued the Jindogae 1 alert," the official said on the condition of anonymity.

"After searching the area, the military found no signs of intrusion from the North's military and lifted the alert," the official said.

Tensions on the Korean Peninsula remain high after North Korea angrily responded to the latest U.N. sanctions to punish the North for conducting its third nuclear test last month. (Yonhap News)

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