Chinese customs officers in Changbai inspect a truck of minerals from North Korea in this photo from the website of the General Administration of Quality Supervision, Inspection and Quarantine of China.

China is stepping up radiation checks to prevent contaminated North Korean-made products from getting through the border, according to the Chinese inspection and quarantine administration. Already last year, China returned about 666 tons of minerals which surpassed the radiation safety limit to the North.

The move comes in the wake of the North's latest nuclear test, but the Chinese government has not commented on the reasons for tougher inspections.

The General Administration of Quality Supervision, Inspection and Quarantine of China in its website last Tuesday said the Changbai customs office on the North Korea-China border conducted radiation checks on 1,227 cases of minerals imported from the North in 2012, and sent back 665.96 tons which emitted more radiation than permitted.

Changbai sits across the Apnok River from Hyesan in North Korea, only about 100 km from the nuclear test site in Pyunggye-ri.

The office did not state the total amount of minerals China imported from the North in 2012 or in January and February this year. But it has boosted equipment to detect radiation with the help of the Jilin provincial inspection and quarantine bureau.

Chinese customs officials conduct inspections jointly with their North Korean counterparts on the bridge over the Apnok River, apparently in order to smooth ruffled feathers in the North.

A foreign affairs and security expert in South Korea said the move "seems to be aimed at preventing proliferation of North Korean nuclear weapons. It's highly likely that China is doing this after consultations with the U.S."

South Korea's Economy Ministry said Seoul is only conducting radiation checks of specific minerals "imported from certain risk countries," suggesting it is unusual for China to check all mineral imports from the North.