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 deadliest 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."

First Outbreak
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.

A government spokesman said Chancellor Angela Merkel was setting up an E. coli task force and has talked to Spanish Prime Minister Jose Luis Rodriguez-Zapatero about the impact on Spanish farmers, who were initially blamed for contamination.

The outbreak has put strains on trade relations, with Russia drawing EU criticism after banning raw vegetable imports from Europe and accusing Brussels of failing to handle the crisis.

Spanish officials have said they might seek compensation from Germany after German officials went back on initial reports that the source might be cucumbers imported from Spain.

Beyond Germany, people have also become ill in Austria, the Czech Republic, Denmark, France, Netherlands, Norway, Spain, Sweden, Switzerland, Britain and the United States.

"All these cases except two are in people who reside in or had recently visited northern Germany during the incubation period for the infection -- typically 3 to 4 days post-exposure -- or in one case, had contact with a visitor from northern Germany," the WHO said in a statement.

Worst Ever
Robert Tauxe of the U.S. Centers for Disease Control and Prevention, which has been working with German health officials since last week, said the strain was likely the most deadly yet in terms of the number of deaths recorded.

"I believe it is," he told Reuters. He said it was unclear how the bacteria became so resistant.

The European Union's envoy to Moscow said Russia's ban on EU produce was unjustified and contradicted World Trade Organization rules.

Fernando Valenzuela also repeated the call from the 27-nation bloc for the lifting of the ba