HUSHAN, China - China has been building a massive barbed wire and concrete fence along parts of its border with North Korea in the most visible sign of Beijing's strained ties with its once-cozy communist neighbor.

Scores of soldiers have descended on farmland near the border-marking Yalu River to erect concrete barriers 8 to 15 feet tall and string barbed wire between them, farmers and visitors to the area said.

Last week, they reached Hushan, a collection of villages 12 miles 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,” said a farmer, who only gave his surname, Ai. “I assume it was built to prevent smuggling and illegal crossing.”

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 was approved in 2003.

Experts and a local Hushan official, who requested anonymity because of the sensitivity of the project, said the military was in charge of the building.

A Defense Ministry spokesman, Ye Xing, 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 U.S.-led U.N. forces in the Korean War, China left their border lightly guarded, deploying most of its forces in the northeast toward its enemy, the Soviet Union.

But the border 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 northeast China, fording the Yalu and Tumen rivers or walking across the ice in winter.

Professor Kim Woo-jun, of the Institute of East and West Studies in Seoul, said China built wire fences on major defection routes along the Tumen River in a project that began in 2003, and since September this year, China has been building wire fences along the Yalu River.

“The move is mainly aimed at North Korean defectors,” Kim said. “As the U.N. 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. That treaty wasn't reported to the United Nations and therefore does not apply to a third country, such as South Korea.

China is concerned that South Korea may claim a different border after absorbing or unifying with the North.

Reporters who visited the border area in the past week saw about 1,600 feet of newly erected barbed wire fence north of Dandong, mainly along river banks and occasionally broken up by mountain areas or military guard posts.

A duck farmer in Hushan, who would only give his surname Han, said that soldiers began putting up the fence near his farm on Oct. 9 - the same day that North Korea claims to have carried out an underground nuclear test.