The Chinese government has mobilized 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 U.S. 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.