DANDONG, China • China has ramped up security along its border with North Korea, installing new surveillance cameras, deploying extra security forces and operating radiation detectors as it braces itself for a potential crisis.
Bellicose rhetoric between Washington and Pyongyang has raised fears in China of a conflict that could send millions of North Korean refugees across the 1,420km border, and of nuclear fallout that could hit Chinese towns.
While the authorities have been coy about preparations, residents have seen an increase in patrols along the frontier.
Radiation monitors were running in border towns, and locals said interactions with North Koreans have been discouraged.
A red banner tacked to a border fence in Dandong - a major trading hub separated from North Korea by the Yalu River - had a Cold War-like message to residents: "Citizens or organisations who see spying activities must immediately report them to national security organs."
Outside Dandong, new checkpoints dot the road running along the Yalu River. Locals said they were installed in October.
"The North Koreans very likely are flying a patrol" along the Yalu River, said Mr Rick Fisher of the International Assessment and Strategy Centre. "They want to see what they can on the Chinese side" and deliberately "raise Beijing's alarm".
"Before, the North Koreans came to our side to fish. Now they don't dare," said Mr Zhang Fuquan, at his fish farm on the Chinese side of the Yalu River. "The army patrols and watches."
On the opposite bank, North Korean soldiers peered out from turquoise watchtowers, and at least one warplane surveilled the territory from above.
Experts said the aircraft was a Stalin-era Ilyushin Il-28 light bomber or a Chinese copy.
"The North Koreans very likely are flying a patrol along the Yalu," said Mr Rick Fisher, a fellow at the International Assessment and Strategy Centre, a US-based think-tank. "They want to see what they can on the Chinese side" and deliberately "raise Beijing's alarm".
Relations between China and North Korea have deteriorated as Beijing has backed a series of United Nations sanctions to punish its reclusive ally over its missile and nuclear tests.
In a previously unthinkable meeting, top United States diplomats and military officials told their Chinese counterparts last year about US plans to send troops to North Korea and secure its nuclear weapons in case the regime fell.
Five of Pyongyang's six nuclear tests have been carried out under Mount Mantap at Punggye-ri, some 80km from the border with north-east China, where citizens felt the accompanying earthquake.
Some Chinese and foreign scientists worry that the 2,200m peak suffers from "tired mountain syndrome" and could collapse if further nuclear tests are carried out.
In Lagushao village, Agence France-Presse reporters saw a "radiation environment automatic monitoring station" housed inside a hut. Professor Guo Qiuju at Peking University said the station was capable of detecting radiation coming across the border.
"If the monitoring stations show any abnormalities, we will immediately alert citizens," said Prof Guo.