North Korean leader Kim Jong-un visited a military unit near the tense inter-Korean border earlier this month when he was absent from public appearances for 15 days, government sources said Tuesday.

In early August, Kim made an inspection not covered by the state media at the front-line unit, some 1 kilometer away from South Korea's general outpost in the central section of the border, the sources added.

It is not known what activity the North's leader made there, but South Korea's military is viewing the inspection as an unusual, undisclosed move, bracing for North Korea's possible tactical provocations, according to the sources.

The supervision was seen as being conducted sometime before Aug. 14 when Kim inspected the command post of the Strategic Force.

At that time, he delayed a plan to fire four missiles toward the US territory of Guam.

For 15 days, he had not shown up in public since July 30 when Kim attended a banquet celebrating the country's test of an intercontinental ballistic missile days earlier.

A military official said that the border unit inspected by Kim is surrounded by mountainous areas and streams, making it easier for the North Korean military to make surprise provocations.

South Korean and US troops kicked off 10-day joint military drills Monday, which Pyongyang has denounced as a rehearsal for a northern invasion.

Tensions between Pyongyang and Washington heightened after US President Donald Trump and the North Korean leadership traded bellicose rhetoric, with both floating talk of military options.

Kim Jong-un last week held off on threats of "enveloping" missile strikes near Guam, but he vowed to make an "important" decision if the US continues its "extremely dangerous reckless actions" on the Korean Peninsula, apparently referring to the allies' drills. (Yonhap)