Nineteen containers from Japan showing traces of radioactivity have been intercepted in the Dutch port of Rotterdam, Dutch health authorities said on Tuesday.

Five of the 19 containers showed radio active levels above allowed standards, the Food and Consumer Product Safety Authority (nVWA) added.

"We have intercepted 19 containers," nVWA spokeswoman Marian Bestelink told AFP.

"We let 14 go because they showed contamination levels far below the allowed standards," of four becquerel per square centimetre, she said.

A becquerel (Bq) is the international unit used to measure radiation given off by a source.

"Five other containers were isolated because the contamination was above permittable levels," the nVWA said in a statement.

After investigation it showed the contamination level of one of the containers to be at an average of 6Bq per square centimetre.

Contamination levels on the other four containers were still being investigated, the nVWA said.

The containers would be cleaned and another reading taken. They would be released as soon as levels dropped below permittable levels.

The European Union decided on April 15 to strengthen controls to measure radio activity on board ships arriving from Japan after the tsunami and subequent nuclear disaster at Fukushima on March 11.

"Containers from Japan will be controlled for as long as necessary," the nVWA said.