BERLIN - Germany halted sales of poultry, pork and eggs from more than 4,700 farms Friday after animal feed was found to be contaminated with cancer-causing chemicals. As German authorities sought the source of the tainted feed, other nations rushed to figure out if any other food was contaminated.

South Korea and Slovakia on Friday banned the sale of some animal products imported from Germany, while authorities in Britain and the Netherlands were investigating whether products containing German eggs -- like mayonnaise -- were safe to eat.

Prosecutors in the northern state of Schleswig-Holstein have launched an investigation into the German firm Harles & Jentzsch GmbH on suspicion that it knew of possible dioxin traces but failed to alert authorities.

Test results released Friday on animal feed produced by the company showed it contained more than 77 times the approved amount of dioxin.

The scandal broke after regular random testing revealed excessive dioxin levels in eggs from chickens in the west of the country earlier this week. More than 8,000 chickens were ordered slaughtered, and tainted food fears spread to Germany's famous pork.

Germany's Agriculture Ministry said Friday it had no immediate reports of health problems connected to the contaminated food, but it was stopping the sale of products from certain farms as a precaution until more tests can be carried out. About 1 per cent of the country's farms have been affected.

"This strategy is resulting in a high number of closed farms, which in the course of testing and clarification in the coming days will be reduced," Agriculture Minister Ilse Aigner said.

Authorities believe some 150,000 tons of feed produced by Harles & Jentzsch for poultry and swine containing industrial fat has been fed to livestock across Germany. The fat contains dioxins and should not have been in the food.

Dioxins are contaminants that typically result from industrial combustion and other chemical processes. Exposure to dioxins at high levels is linked to increased incidence of cancer.

Test results are expected shortly on whether traces of dioxin have been found in milk or meat in two of the German states where the contaminated feed was delivered to farms, Agriculture Ministry spokesman Holger Eichele said.

He said the most recent tests on eggs from farms where livestock is known to have consumed the contaminated feed, "two-thirds have been clean and about one-third have been right on the border of what is considered dangerous."

Authorities are trying to determine how long the contaminated feed has been in circulation.

Chris Elliott, an expert in food safety at Queens University, Belfast, said so far the danger to consumers appeared to be limited.

"The concentrations detected in this case are above the legal tolerance limits, but only just. That tells you that the potential risk of harm from these eggs is very low," Elliott said.

In Brussels, the European Commission said it was "closely monitoring the situation with the German authorities."

In 1999, dioxin from motor oil was mixed into animal feed in Belgium, leading to widespread import bans and food being pulled from the market. The scandal prompted the European Union to establish maximum levels for dioxins in livestock feed in 2001.

German farmers are demanding compensation for losses they are estimating at up to euro60 million ($79 million) a week, but Eichele said it was still too early to determine the overall damage from the tainted feed.

"We first need to find out what led to this," Eichele said. "It needs to be cleared, then we need to see how severe the damage is and then how we can best help those farmers."