China is building a massive fence along parts of its border with North Korea in the most visible sign of strains in Beijing's once-cozy ties with its communist neighbor.
Scores of soldiers have descended on farmland near the border-marking Yalu River to erect concrete barriers up to 4m tall and string barbed wire between them, farmers and visitors to the area said.
Last week, they reached Hushan, a group of villages 20km inland from the border port of Dandong.
"About 100 People's Liberation Army soldiers in camouflage started building the fence four days ago and finished it yesterday," a farmer said.
Though the fence-building appears to have picked up in the days following North Korea's claimed nuclear test last week, experts said the project had been approved in 2003.
A defense ministry spokesman declined comment, saying he was not authorized to release information on border security.
The fence marks a noticeable change in China's approach to North Korea.
In the decades following their shared fight against US-led UN forces in the Korean War, China left the border lightly guarded, deploying most of its forces in the northeast toward its enemy, the Soviet Union.
The border, however, became a security concern for Beijing in the past decade, as North Korea's economy collapsed and social order crumbled in some places. Tens of thousands of refugees began trickling across the border into China.
Professor Kim Woo-jun at the Institute of East and West Studies in Seoul said China built wire fences on major defection routes along the Tumen River starting in 2003, and since last month, China has been building wire fences along the Yalu River.
"The move is mainly aimed at North Korean defectors," Kim said. "As the UN sanctions are enforced ... the number of defectors are likely to increase, as the regime can't take care of its people ... I think the wire fence work will likely go on to control this."
But he said he also believes that Beijing wants to firmly mark its border with the North along the two rivers.
Kim said China and the North drew their border in a secret treaty, which wasn't reported to the UN and therefore does not apply to a third country, like South Korea. China may be concerned that South Korea could claim a different border after absorbing or unifying with the North.