Tensions have thawed recently, but things are still tense along China's border with North Korea.
Life along North Korea's northern borders with China and Russia stands in stark contrast with the southern frontier it shares with South Korea.
While the southern border is marked by the heavily fortified demilitarized zone, long stretches of the border with China only have some surveillance equipment or barbed-wire fences in various stages of disrepair.
Reuters correspondents Damir Sagolj and Sue-Lin Wong traveled along North Korea's northern border in November and March this year. They peered across the frozen rivers that separate the countries, catching glimpses of life in the hermit kingdom.
But they also encountered strict Chinese security and areas where China's military seemed to be up to something — activity that observers said was in preparation for turmoil in North Korea.
"Border regions anywhere in the world are sensitive, but it's particularly sensitive up here because the river is narrower and historically there has been a lot of interaction between the two sides," said Li Zhonglin, a China-North Korea specialist at Yanbian University.
Below, you can see what life is like on North Korea's remote northern frontier:
"Nowhere in the world is there such a difference between what life looks like on opposite sides of the river or the fences that separate the two countries," Sagolj said.
Here, people in Tumen, China — one of the more closely watched and heavily guarded spots on the border — have their picture taken with North Korea in the background, November 25, 2017.
The 880-mile border between North Korea and China was challenging, Sagolj said. In between the fortifications in the more populated areas "is mostly darkness on the North Korean side and long, cold sections of emptiness on the Chinese side."
Here, a North Korean woman walks between houses in Hyesan, seen from the Chinese town of Changbai, November 23, 2017.
"Just outside Linjiang, as the road started winding towards the mountains, we spotted people waist-deep in the river — a group of maybe 20 men wearing weird rubber orange suits, as if they were in a very old and very cheap sci-fi movie," Sagolj said. "We stopped immediately and I got my longest lens from the back of the car."
"Locals said they were North Koreans — I saw military guards on the other side who closely monitored their work," he added. "As I was frantically taking pictures, Sue-Lin asked the Chinese what was going on. The locals said they were looking for gold."
It was almost impossible to confirm the story on the spot, Sagolj added, but "experts and documents confirmed that people do indeed search for gold in the Yalu river, so the gold story could be true. We asked North Korean officials about claims 'Office 39,' which procures luxury goods for North Korea's ruling Kim family, has a hand in the gold mining, but they didn't respond."
The isolation on the North Korean side of the border was broken only occasionally. "Driving along those frozen fields, past villages with rickety houses or small, dirty industrial towns, I only rarely saw people actually interacting with each other," Sagolj said, "once two women fighting, the other time, three beautiful little girls in new red boots playing while they fetched the water."
While the border is porous and in some areas unguarded, China is vigilant and perhaps growing more so. "Besides what we couldn't see with naked eye — thermal sensors, satellites and who knows what else — occasional cameras are mounted above and alongside the road, and strategically placed checkpoints are manned by heavily armed military," Sagolj said.
China is particularly concerned about North Koreans crossing the border into China — a task made easier when the two rivers along the frontier freeze over. Despite improved relations between China, North Korea, and the US, "locals say that over the past six months, China has stepped up military patrols here," Wong said. "Media reports in South Korea and elsewhere say China has been bolstering its defenses along the border — some have said as many as 300,000 troops have been added."
That would be an increase from 100,000 troops in August 2017. China has military bases along the border, and Chinese security personnel kept a close watch on Sagolj and Wong, preventing them from entering an area in the middle of the border. "We have secret military installations here," a soldier told Wong in November. A declassified CIA document from May 1970 said China started building personnel trenches — possibly with forward firing positions facing the border — in 1969.
Diplomatic sources told Wong that China has made extensive plans to respond should war break out in North Korea and was likely to intervene in some way. In late 2017, a leaked document showed what appeared to be plans by a Chinese state-owned company to build refugee camps along the border. Locals told Wong they saw no signs of construction, but had little doubt such plans existed.
Residents of the area said abandoned homes near the border could be used as shelters if North Koreans were to flee en masse. But academics in China said Beijing is likely to see a such an exodus as a military problem rather than a humanitarian one.
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