Researchers are still unable to pinpoint the cause of the outbreak that has hit Germany and other European nations, which has infected 1,500 people.

Russia on Thursday extended its ban on vegetable imports from the EU in a bid to prevent a deadly European bacterial outbreak that has left 17 people dead from spreading into the country.

Researchers are still unable to pinpoint the cause of the outbreak that has hit Germany and other European nations, which has infected 1,500 people.

Lyubov Voropayeva, spokeswoman for the Russian Agency for the Supervision of Consumer Rights, told the Associated Press on Thursday that the ban has been imposed immediately for no definite period of time.

The agency’s chief Gennady Onishchenko told Russian news agencies that this “unpopular measure” would be in place until European officials inform Moscow of the cause of the disease and how it is being spread.

“How many more lives of European citizens does it take for European officials to tackle this problem?” he told the state—owned RIA Novosti news agency.

Russia banned fresh imports from Spain and Germany on Monday, warning of a possible spread of the sanction. No fatalities or infections have yet been reported in the country.

The outbreak has hit at least nine European countries, but nearly all the sick people either live in Germany or recently traveled there. Two people who were sickened are now in the United States, and both had recently travelled to Hamburg, Germany, where many of the infections occurred.

Medical authorities appeared late on Wednesday no closer to discovering the source of the infection. Germany’s national health agency said that more than 1,530 people there had been sickened by the dangerous E. coli germ, including 470 suffering from a kidney failure complication that was previously considered extremely rare.

The outbreak is already considered the third—largest involving E. coli in recent world history, and it may be the deadliest. Twelve people died in a 1996 Japanese outbreak that reportedly sickened more than 12,000, and seven died in a 2000 Canadian outbreak.