Five children admitted to hospital in northern France after eating beef burgers infected with a strain of E.coli bacteria are seriously ill, health officials said on Thursday, fanning fears of a wider outbreak.

The officials said the bacteria did not appear to be related to the lethal strain of E.coli that has killed 37 people and made 3,000 ill, most of them in northern Germany.

Privately owned German discount chain Lidl withdrew boxes of the frozen beef patties believed to be behind the French infections. The boxes were sold under the brand "Steaks Country" and had expiry dates of May 10, 11 and 12, officials said.

On Wednesday six children, aged between 20 months and eight years and from different towns in the Pas de Calais region, were taken to a hospital in the city of Lille after suffering bouts of bloody diarrhoea.

One was quickly released, but five are in a "serious condition" and still being treated at the hospital. Three are being treated with hemodialysis, a method of removing waste products from the blood in the case of kidney failure.

Although the outbreak comes on the heels of an E.coli epidemic linked to contaminated bean sprouts which has killed 36 people in Germany and one in Sweden, health officials said the French cases had no apparent link to those in Germany.   

"I hope we can launch a search programme very quickly, we are working on that already with French researchers in order to identify (the origin) and deal with health problems soon," Health Minister Xavier Bertrand said on RTL radio.   

He said the entire production chain for the beef patties would have to be examined and stricter controls would be enforced.   

A spokesman for Lidl said that beef used in the suspect boxes had been bought from French supplier SEB-CERF, based in the northeastern town of Saint-Dizier, which produces some 400 tonnes per week of frozen beef, according to its website.

"The products were made in France, but depending on the expiry date and our suppliers' opportunities, the beef can come from Germany, Italy, France, the Netherlands or even other places," spokesman Jerome Gresland at Lidl told Reuters.   

"We buy the beef through this supplier with a stamp that says it comes from the European Union."    

French health authorities ordered Lidl to withdraw "Steaks Country" boxes with a range of expiry dates after finding a box with a May 10 expiry date at the home of one of the infected children.

The firm withdrew the entire line of frozen beef patties. - Reuters