Germany stepped up efforts Tuesday to trace the source of a bacteria outbreak that has killed 24 amid warnings it might never be found, while the EU pledged greater aid for vegetable farmers hit by the scare.

An employee of the microbiology and toxicology department at the chemical and veterinary examination office Stuttgart runs tests on bean sprouts in the southern German town of Fellbach.

After following several apparently false leads, German authorities zeroed in on organic bean sprouts as the possible origin of the contamination with a highly virulent strain of E. coli.

But initial probes carried out on a farm growing a variety of sprouts in the northern state of Lower Saxony proved negative.

Another lead in Hamburg, the epicentre of the outbreak, had involved sprouts from the same farm found in the refrigerator of a sickened man. But that sample too failed to display any trace of the bacterial strain.

The scare has led puzzled officials to warn consumers off raw tomatoes, cucumbers, lettuce and sprouts, prompting the European Commission to ask EU states for 150 million euros ($220 million) in aid for hard-hit farmers.

European agriculture commissioner Dacian Ciolos urged Germany to quickly pinpoint the driver of the lethal bacterial strain, warning that consumers were losing confidence every day the outbreak remains a mystery.

"Without this answer, it will be difficult to regain the trust of consumers, which is essential for the market to regain its strength," he said at emergency talks in Luxembourg.

Belgian Agriculture Minister Sabine Laruelle estimated losses to EU farmers "in the hundreds of millions of euros" after countries such as Russia banned vegetable imports and European consumers turned their backs on greens.

"European solidarity must rise to the occasion," Laruelle said.

Ciolos came out of Luxembourg talks pledging to write a "substantially" bigger cheque than the initial offer of 150 million euros.

He indicated he would come up with a new figure as early as Wednesday, but warned that it would be difficult to meet demands for full compensation after he proposed to cover 30 percent of losses.

"I am ready to raise this 30 percent level, but I don\'t think that the budget will allow us to reach 100 percent for all goods and all producers," he said, noting that the funds would come from the EU\'s common budget.

Laruelle\'s Spanish counterpart Rosa Aguilar said several nations signed a document calling for payouts of 90 to 100 percent depending on the product, adding that the crisis was costing Spanish growers 225 million euros a week.

Health officials in Hamburg had fingered organic cucumbers from Spain as the culprit last month before backtracking when tests came up negative.

Infections have killed at least 23 people in Germany, according to adjusted official figures released Tuesday. The other fatality was a woman in Sweden who had recently returned from Germany.

The outbreak has left more than 2,300 ill, with symptoms ranging from bloody diarrhoea to, in full-blown cases, kidney failure. The Robert Koch Institute, Germany\'s national health centre, said the rate of reported infections appeared to be tapering off.

Meanwhile German officials defended their decision to issue warnings on vegetables to the public even before full test results were known.

But opposition members have criticised the slow pace of the probe, suggesting that the country\'s federal system was partly to blame because public health matters are handled by both national and state authorities.

A World Health Organisation expert said tracing the path of the outbreak to its source would be particularly tricky because of the time elapsed between contamination and testing.

"Often such an outbreak is caused by a single batch of produce, and by the time you get to sampling, the batch is out of the system," Guenael Rodier, who is in charge of communicable diseases at the WHO\'s Europe division, told AFP.