Dutch customs officials found higher than allowed radioactivity levels in a container that arrived by ship from Japan eight weeks after the Fukushima nuclear accident, a spokeswoman said.

"Traces of radioactivity were detected on the exterior of the container, but none in the cargo itself," customs spokeswoman Gera van Weenum told AFP, adding the levels exceeded the maximum authorised of four becquerel, but were not dangerous.

"There were spots of radioactivity of up to 33 becquerel, but the average was six becquerel," said the spokeswoman.

The contamination was detected yesterday as the container passed through standard security checks at the port of Rotterdam, and was then sent for a second, more thorough inspection before being put on a truck to its final destination, a Dutch importer.

The container was cleaned on its arrival there, after which the importer sent it to First Safety, a Dutch company that specialises in tracing dangerous substances, for a further check. The nature of its cargo was not disclosed.

First Safety head Hans Jansen told AFP an average level of 40 becquerel was measured, and spots of up to 200 becquerel, which "could be dangerous".

The European Union decided last month to boost radioactivity checks on ships from Japan, after a massive quake and tsunami on March 11 damaged the Fukushima Daiichi nuclear power plant northeast of Tokyo, sparking the world's worst atomic crisis since Chernobyl a quarter-century ago.