The WHO advisory came as German and Chinese researchers said they cracked the genetic code of the E. coli strain, which they said in a preliminary analysis was resistant to antibiotics and extremely virulent.
US health officials said Thursday that three people were suspected to have fallen ill from the bacteria after travelling to Germany, where the mystery outbreak has killed 17. Sweden has also reported one victim.
Russia banned vegetable imports from the European Union in response to the scare, in a move the bloc slammed as "disproportionate", as Britain reported its first cases of the outbreak and Spain demanded a payback for its farmers.
|A nurse cares for a patient infected with the virulent E. coli bacteria undergoing dialysis treatment at a hospital in Hamburg on June 1|
There were contradictory accounts as to whether the strain of E. coli was new, or whether Europe was witnessing the first outbreak of a rare but known type of bacteria that officials believe is carried by raw vegetables.
The outbreak was initially blamed on Spanish cucumbers by German officials who later admitted that they were in the dark about its origin.
"It has been seen in sporadic cases and is very rare."
But researchers at a German hospital treating patients felled by the germ sequenced its genome and called it "a new type of E. coli strain" while their Chinese partners dubbed it an "entirely new, supertoxic E. coli strain."
Authorities have also failed to pinpoint the origin of the Enterohaemorrhagic E. coli (EHEC) that has infected more than 2,000 people, 500 of them with full-blown haemolytic uraemic syndrome (HUS), a potentially deadly disease that causes bloody diarrhoea and serious liver damage.
As confusion reigned over how to tackle the scare, Russia said it had blacklisted imports of fresh vegetables from EU countries with immediate effect and slammed food safety standards in the bloc.
Russia's Rospotrebnadzor watchdog said its ban would remain in force until the EU explained how the bug was transmitted.
"This shows that Europe's lauded health legislation -- one which Russia is being urged to adopt -- does not work," consumer watchdog chief Gennady Onishchenko was quoted as saying by the Interfax news agency.
The European Commission demanded an about-face from Russia, whose vegetable imports from Europe amount to around 600 million euros ($868 million) a year.
The United Arab Emirates also banned the import of cucumbers from Germany, Spain, Denmark and the Netherlands over the scare.
Meanwhile Britain said seven people there had been infected with the bacteria, including three British nationals who had recently travelled to Germany and four German nationals.
Three had HUS and the other four suffered bloody diarrhoea, the Health Protection Agency said.
Officials in the northern German port city of Hamburg, the epicentre of the outbreak, had last week cited imported Spanish cucumbers as the source of the contamination.
But tests on two Spanish cucumbers there this week showed that while they carried dangerous EHEC bacteria, it was not the strain responsible for the current massive contamination, whose toll in Germany rose to 17 after the death of an elderly woman in Hamburg overnight.
Prime Minister Jose Luis Rodriguez Zapatero said Spain, already struggling with a weak economy and high unemployment, would seek compensation over the false allegations.
"Yesterday, it became clear, with the analyses carried out by the Spanish agency for food safety, that there is not the slightest indication that the origin of the serious infection is any Spanish product," he said in an interview with Spanish national radio.
Spain will also "seek reparations before the relevant authorities in Europe for the harm sustained," he said, after the European Commission lifted its warning over Spanish cucumbers.
Spain's fruit and vegetable exporters estimate they have lost more than 200 million euros a week as 150,000 tonnes of produce went unsold in a Europe-wide reaction to the outbreak.