Millions of chickens could be culled in the Netherlands over fears of insecticide-tainted eggs, an industry body said as Belgium vowed full transparency Monday about why it kept the scandal secret.

Supermarkets in Germany, the Netherlands, Belgium, Sweden and Switzerland have pulled millions of eggs from the shelves after fipronil, a substance potentially dangerous to humans, was found in them.

Dutch farming organization LTO said that several million hens may need to be culled at 150 companies in the country, with 300,000 having already been killed.

An LTO spokesman said late Sunday that they "had to be eliminated because of contamination".

Belgium's agriculture minister meanwhile said he had ordered the country's food safety agency to report by Tuesday on why it failed to notify neighboring countries until July 20 despite knowing about fipronil contamination since June.

Denis Ducarme said in a statement he had told the food safety agency (AFSCA) to produce a "report on the circumstances of the agency's actions since the first information it received about the fipronil problem."

Facing pressure from Germany and the Netherlands, Ducarme promised "complete transparency."

He said he would speak by telephone with his counterparts in the coming days.

Ducarme added that products from 57 Belgian egg producers -- around a quarter of the country's -- had been blocked as a preventative measure.

Belgian officials admitted on Saturday they had kept the problem under wraps and failed to trigger the EU's international food safety alert system but said it was because of a fraud probe.

European Commission spokeswoman Anna-Kaisa Itkonen said she could not comment on the Belgian delay "because it's an ongoing criminal investigation."

Germany has demanded an explanation from Belgium about why the issue was kept covered up.

Fipronil is commonly used in veterinary products to get rid of fleas, lice and ticks. But it is banned from being used to treat animals destined for human consumption, such as chickens.

In large quantities, the insecticide is considered to be "moderately hazardous" according to the World Health Organization, and can have dangerous effects on people's kidneys, liver and thyroid glands.

It is believed the toxic substance was introduced to poultry farms by a Dutch business named Chickfriend brought in to treat red lice, a nasty parasite in chickens.

Dutch and Belgian media reports that the substance containing the insecticide was supplied to Chickfriend by a Belgian firm have not been confirmed.