South Korea is tightening customs clearance for coal waste imports from Japan, citing concerns they may be contaminated with radioactive materials. 

The Environment Ministry said Thursday it will strengthen radioactivity checkups on Japanese coal imports. 

So far, local importers have measured radiation levels themselves and reported the results to the authorities. From now, the ministry will check every coal waste shipment and take necessary countermeasures if there's a problem with contamination. 

According to the Korea Customs Service, 99-point-nine percent of coal waste imports, which are used to make cement, came from Japan over the past 10 years. 

Seoul is taking the measure for the first time amid its conflict with Tokyo over Japan’s recent trade restrictions.