A trader in a market in St. Petersburg. Russia banned imports of raw vegetables from the European Union on Thursday because of a deadly E.coli outbreak centered in Germany. Alexander Demianchuk / Reuters

BERLIN/LONDON - Racing to curb the spread of a killer food bug, Germany set up a national task force on Friday to hunt down the source of a highly toxic strain of E.coli that killed 17 people and sounded alarms around the world.

Russian Prime Minister Vladimir Putin, engaged in a trade row with the European Union after Moscow banned imports of raw fruit and vegetables from the bloc, heightened the drama, saying he would not "poison" Russians by lifting the embargo.

Repeating warnings to Germans not to eat salad vegetables - rattling farmers and stores just as they hit high season - health officials said they recorded 199 new cases of the rare, highly toxic strain of the infection in the past two days.

That took the total of those infected since it was detected in early May to 1,733 - making it possibly the worst ever outbreak - and suggesting it was spreading as fast as ever.

Scientists struggled to pinpoint the contamination, assumed to be poor hygiene at a farm, in transit, a shop or food outlet.

Health institutes across Europe have tried to reassure the public by stressing that E.coli, a frequent cause of food poisoning, can generally be tackled by washing vegetables and by washing hands before eating to reduce the risk of bacteria being passed on from the feces of an infected person.

The resistance of the strain to some antibiotics and the failure to find the source of the outbreak, made harder by the nature of salads to include a variety of produce from different producers, has raised concerns, however.

Responding to EU calls that Russia lift Thursday's ban on imports and respect the principle of free trade, Putin said: "We cannot poison our people for the sake of some spirit."

Germany is at the center of the outbreak but people have also become ill in 10 other European countries and the United States, probably from eating lettuce, tomatoes, cucumbers or other raw salad vegetables in Germany. The region around the northern city of Hamburg is at the epicenter of the sickness.

The World Health Organization said the strain was a rare one, seen in humans before, but never in this kind of outbreak.

E.coli bacteria themselves are harmless. But the strain that is making people sick in Europe has the ability to stick to intestinal walls where it pumps out toxins, sometimes causing severe bloody diarrhea and kidney problems.

Reuters