The World Health Organization (WHO) on Thursday said it has identified "a rare strain" of E.coli as the cause for an outbreak which has led to at least 18 deaths and sickened thousands in Europe.

"This strain has never been seen in an outbreak situation before," WHO spokeswoman Aphaluck Bhatiasevi said, adding that the more tests were currently being conducted.

Health authorities started hunting for the source of the bacteria, after Germany blamed the outbreak on Spanish cucumbers, prompting an angry response from Madrid as it threatened to seek compensations from Berlin.

In the latest statement posted on its website, the WHO said that the cause of the outbreak was still unclear.

The European Union's disease watchdog, the European Center for Disease Prevention and Control also said on Thursday that the source of the outbreak was still under investigation, quoting contaminated food as the most likely vehicle of infection.

But the center said currently there is no indication that raw milk or meat is associated with the outbreak. Health authorities have previously warned the public not to consume raw cucumbers, tomatoes and lettuce.

The development came as a heavy blow to Europe's vegetable industry, especially after Russia banned all import of fresh vegetables from EU member states from Thursday.

According to WHO statistics, the E.coli outbreak has so far sickened more than 1,600 people, among them 499 suffered from a kidney failure complication.

A total of 18 people have reportedly died from the infection, mostly in Germany.