North Korea appears to be fortifying its border in the Demilitarised Zone (DMZ) with the South, days after a soldier defected by running across.
A US diplomat to South Korea has tweeted a picture showing workers digging a trench.
The defector was shot multiple times by border guards from the North at the spot last week.
South Korea has handed out medals to its soldiers who helped rescue the wounded defector.
Marc Knapper, chargé d’affaires at the US embassy in Seoul tweeted a picture of the scene after a visit to the DMZ.
Another diplomat visiting the same spot told the Reuters news agency he had also seen several workers digging a trench.
On 13 November the North Korean soldier drove up to the border, got out of the car and sprinted across.
Other soldiers from the North running after him fired at the man, seriously wounding him.
From footage later released by the UN, it appears the defector could have driven across if his vehicle had not broken down.
The footage showed no visible fences, concrete bollards, or conspicuous obstacles that one might expect at the border of two countries which are technically still at war.
Marking a line
Analysis: Paul Adams, Seoul
What the ambassador’s photo appears to show is an effort by the North Korean military to avoid the mistakes of 13 November.
On Wednesday, the UN accused the North Koreans of two violations of the 1953 Armistice Agreement which brought the Korean War to an end. One involved shooting weapons across the “Military Demarcation Line” that separates North and South Korea.
The other occurred when one of the defector’s North Korean pursuers briefly crossed the invisible line. The CCTV footage released on Wednesday showed him running forward into the South, but almost immediately retreating back on to the North Korean side. It seemed he realised, or was told by his colleagues, what he had just done.
In digging up the ground where the incident occurred, the North Korean authorities may be trying to make sure their own soldiers know where the line is.
The 250km (155-mile) long DMZ is heavily fortified but the Joint Security Area (JSA) is the only place in where soldiers from both sides face each other.
Located in the village of Panmunjom, it’s a popular tourist attraction for South Koreans.