China has reinforced fences and patrols along parts of the border with North Korea in an apparent bid to ward off a growing stream of refugees from its impoverished neighbour, a report said.

Four-metre-high (13.2 foot) fences topped with barbed wire are being erected along the Yalu River around the Chinese city of Dandong facing the North's northwest city of Sinuiju, Yonhap news agency reported late Tuesday.

The river, which marks the border in the North's northwest, is a popular escape route for those seeking to flee permanently or to find food in China.

Previously the border was only marked by a three-metre-high fence which "anybody could cross if they really wanted", said a Dandong resident quoted by Yonhap in its report from the city.

Work on the new fence sections, which currently stretch for about 13 kilometres (8 miles), began last November and is continuing, Yonhap said. New patrol posts were being built on higher ground to improve monitoring.

"It's the first time such strong border fences are being erected here. Looks like it's related to the unstable situation in North Korea," the resident was quoted as saying.

A growing number of North Koreans have fled the country amid continuing food shortages. Almost all cross first to China, which repatriates those whom it catches as economic migrants.

Refugees often travel on to third countries before eventual resettlement in South Korea.

More than 20,000 North Koreans have arrived in the South since the end of the 1950-1953 war, the vast majority in recent years.