The World Health Organization says that a deadly outbreak of E. coli bacteria that has killed 17 Europeans is a new strain that has never been seen before.

The U.N. agency said Thursday that preliminary genetic tests suggest that the strain is a mutant of two different E. coli bacteria with lethal genes. A food safety expert at WHO, Hilde Kruse, said that the strain has “various characteristics” making it “more virulent and toxin-producing.”

More than 1,500 people in nine European countries have been sickened by the rapid spread of the bacteria, with all but one of the deaths and hundreds of the illnesses occurring in Germany. Health officials have been unable to find the cause or origin of the outbreak.

With the uncertainty, Russia said it is banning the import of all fresh vegetables from the European Union, an action the EU immediately called “disproportionate.” The EU, which exported $853 million worth of vegetables to Russia last year, said it would seek an explanation from Moscow.

Russia said that vegetables already imported from EU countries would be seized. The chief of Russia's consumer protection agency, Gennady Onishchenko, urged his countrymen to “forgo imported vegetables in favor of domestic products.”

He said the E. coli outbreak shows that Europe's health legislation does not work. Germany initially pointed to cucumbers from Spain as a possible source of the contamination, but further tests showed that those vegetables were not the cause of the outbreak.

The erroneous conclusion angered Spanish officials. Prime Minister Jose Luis Rodriguez Zapatero said Spain would “seek reparations” from the “relevant authorities” in Europe for the damages its vegetable growers have suffered.

Germany's national disease center says the outbreak started nearly two weeks before the first infections were reported in mid-May. The infection attacks the victims' kidneys, sometimes causing seizures, strokes and comas.

Vegetable growers across Europe say they are suffering major economic losses as the mystery goes unsolved. Spain says it is losing $288 million a week because of import bans and weak demand for the produce, while the Netherlands says it is losing $43 million.

The president of Spain's produce export trade group said that “almost all Europe” had stopped buying Spanish vegetables and fruit.