Italian customs officials said Monday they had seized more than six tonnes of chemical "precursors" from China that could have been used to make narcotics worth 630 million euros ($678 million).

The seizure was made at Milan's airport during an inspection of goods, and included enough chemicals to produce more than 63 million MDMA pills, they added.

An Italian entrepreneur from Milan was placed under investigation and two Chinese nationals were arrested in the Netherlands as part of investigations carried out with European judicial agency Eurojust, said Italian customs.

After being analysed in a laboratory, powder seized that purported to be polyester powder coating turned out to be PMK -- a precursor product of ecstasy.

The cargo was intended to pass through a company with headquarters in Milan before being shipped off to the Netherlands.

Dutch police officers contacted by Italian colleagues identified the intended final destination and seized dozens of kilos of PMK, ketamine, and hashish.