Image copyright AFP/Getty Image caption No clear source for the outbreak has yet been identified

Five people have now died in a major E. coli outbreak in the US involving romaine lettuce, with 197 cases reported across 35 states.

The Centers for Disease Control and Prevention (CDC) said 25 more people had been affected since its last report on 16 May.

Two of the victims were from Minnesota, with the other three from Arkansas, California and New York.

It is the largest US outbreak of E. coli since 200 people fell ill in 2006.

According to the latest statement from the CDC, many of the people affected fell ill two to three weeks ago, when the contaminated lettuce was still on shop shelves.

Romaine lettuce from the Yuma growing region in Arizona is thought to be the source of the latest outbreak, although the Food and Drug Administration says no single grower, distributor or region can account for the spread.

An investigation is ongoing.

The CDC said that some of the affected people had not eaten lettuce, but had contact with others who had fallen ill.

When eaten, it can cause diarrhoea, vomiting and even kidney failure in severe cases.

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Of the infected people, 89 have been hospitalised, and 26 have developed a kidney failure type known as hemolytic uremic syndrome.

Canada's Public Health Agency has also recorded six cases of E. coli "with a similar genetic fingerprint" to the US infections.

The E. coli outbreak began in April and has spread across the US.

California and Pennsylvania are recording the most cases.