DANDONG, China (Reuters) - Just over half a century ago, Chinese soldiers fought and died alongside North Koreans battling U.S.-led armies in the 1950-53 Korean War.
Today, Chinese troops are busy erecting a barbed-wire fence along the once-friendly border. It is unclear whether this is intended to stem the slow tide of refugees, who now slip across from the North, or a flood in the event North Korea collapses.
Tensions are unusually high around Dandong, the border town on the Yalu River from which some of China's aid passes over the Friendship Bridge to North Korea, Beijing's erstwhile ally.
The concern in Dandong stems from North Korea's announcement this week that it had tested a nuclear device, a move China quickly condemned in unusually strong terms.
Beijing traditionally shies away from talk of cutting off its flow of food and energy to Pyongyang, fearing this could hasten the country's disintegration and destabilise northeastern China.
China now seems to be preparing for all possible outcomes.
This week, groups of troops could be seen furiously shovelling dirt down riverside embankments to secure a row of concrete fence poles standing upright next to the river that marks much of the border.
One one stretch of the Yalu, North Korean is just a few metres away.
Gruff soldiers in fatigues brush away questions, but Dandong residents are a bit more forthcoming.
"This is the first time they have put a fence here," said one weatherbeaten farmer, selling peaches and nuts near the sweating soldiers. The work began on Sunday, a day before the North's purported nuclear test, he said.
"They say the North Koreans are preparing land over there to billet troops," the farmer added.
It was unclear how far the fence extended, but sections could be made out through the undergrowth over several kilometres outside Dandong.
COWS AND CAMERAS
Further along the road, down a bumpy track, another section of fence that locals say went up a few months ago runs through one of the rare areas in this sector where the two countries share a land border.
It appeared incomplete. There was a large gap where anyone could cross, with a solitary stone border marker. Cows appeared to outnumber soldiers.
No Chinese or North Korean troops were visible, but a large pole on China's side topped with a surveillance camera could be seen, keeping a close watch on the quiet scene.
"Who would want to cross over there?" said Liu Yuancun, 38, when asked if she had ever been to North Korea. "It's like China 20 years ago. They're poor and they don't have anything to eat."
Farmhouses in Dandong are sturdy, and many have satellite television dishes outside.
By contrast some of the North Korean homes, viewed from Chinese boats that take tourists right up to the river bank, have plastic bags instead of glass covering their windows.
People in Dandong have traditionally enjoyed a fairly close, warm relationship with North Korea.
Dandong is home to the Museum to Commemorate the War against U.S. Aggression -- China's take on the Korean War -- and what is left of a bridge bombed by U.S. forces. It has become a tourist attraction complete with posters condemning "imperialism".
Shops across the small but bustling city sell North Korean ginseng, cigarettes and other curios. Bilingual Chinese-Korean signs are common.
After North Korea's apparent nuclear test, many in Dandong are not so sure about the friendliness of its neighbour, whose ties with China were long described as "close as lips and teeth".
"This nuclear test can only bring trouble for us and slow our development," complained Li Shijie, peering through the mist at the North Korean city of Sinuiju on the other side.
"They are a terrorist country."
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