This picture taken from North Korean TV and released by South Korean news agency Yonhap on January 6, 2016 shows North Korean leader Kim Jong-Un signing a document of a hydrogen bomb test in Pyongyang. (AFP Photo)

© Thomson Reuters 2016

The Chinese government has mobilised more than 500 people to carry out round the clock monitoring for radiation along the border after North Korea said last week it had carried out its fourth nuclear test, but nothing abnormal has been found.North Korea's test on Wednesday angered both the United States and China, which was not given prior notice, though the US government and weapons experts doubt the North's claim that the device it set off was a hydrogen bomb.China's environment ministry said in a statement on Monday more than 500 people were involved in monitoring for radiation, including about 350 people along the border itself, along with 37 fixed monitoring stations and 14 mobile ones.It showed pictures on its website of a radiation monitoring vehicle driving along a snowy road along the border, and an official standing in the snow looking at technical equipment.But the ministry had already "basically ruled out" the possibility the test would have any radiation impact upon China, and nothing abnormal had yet been found, it said.The ministry was testing air, soil and snow samples, and would continue to test and maintain their current emergency response mechanism.Residents close to the border last week told Reuters they were concerned about the environmental effect of the test.China is North Korea's main economic and diplomatic backer, and has repeatedly expressed anger with Pyongyang for its nuclear tests and other sabre-rattling.China has signed up for tough United Nations sanctions on North Korea and insists it follows them, including carrying out border inspections, but it also provides large amounts of aid off the books to Pyongyang, experts and diplomats say.China fears North Korea's nuclear programme destabilises its neighbourhood and gives the United States a pretext to send weapons and forces to the region.But many Chinese experts fear that if China pulled back support for its neighbour, it could destabilise the country and send a flood of refugees into China.