The environment ministry announced on its social media account that it began "emergency radiation monitoring" along its northeastern border shortly before noon.
The emergency response was set at "level 2", the second- highest grade on a four-tier system.
It did not indicate whether any radiation had been detected.
The decision follows what North Korea has described as the successful detonation of a hydrogen bomb.
The resulting explosion was considerably larger than previous tests and was felt by residents in Chinese cities hundred of kilometres from the North's border.