A sample of iceberg lettuce from the food supplier Taylor Farms has tested positive for the parasite that caused an explosive diarrhoea outbreak affecting thousands of people in the US, officials said.

The Food and Drug Administration (FDA) said that Taylor Farms, which supplies the Taco Bell food chain, had confirmed the sample was taken from a product not part of its current recall announced on Friday.

The agriculture company announced on Friday that it was voluntarily removing all iceberg lettuce from the US market sourced from central Mexico and issued a list of affected products.

No deaths have been reported but at least 94 people have been hospitalised due to cyclosporiasis infections.

A recall differs from a voluntary product removal in that it requires a company to notify customers and recover affected products from commerce, making it a broader and more costly corrective action.

The FDA said it collected the sample during targeted surveillance of the import as part of its investigation into the outbreak.

"Taylor Farms has confirmed that this positive product is not part of their current recall," the FDA said. "They are currently working to identify whether any part of this implicated lot is available in commerce or in consumers homes."

The infections were first detected on 13 May, according to the FDA.

Symptoms of the foodborne illness can take up to two weeks to present. They include watery diarrhoea that lasts for days, sudden weight loss, and a loss of appetite.

Thousands of people have been infected with Cyclospora linked to iceberg lettuce supplied by Taylor Farms and sold at Taco Bell locations in at least five states, according to the Centers for Disease Control and Prevention (CDC).

The CDC advises not to eat shredded iceberg lettuce served at Taco Bell locations in Indiana, Kentucky, Michigan, Ohio, and West Virginia.

The fast food chain said it had completed its removal of affected Taylor Farms lettuce from its restaurants.

In a statement, Taylor Farms said it has "stopped receiving product from the implicated lot, suspended distribution of the iceberg lettuce from Central Mexico, notified [its] customers and [is] continuing to work with the FDA, CDC, and state authorities".

The lettuce was recalled in 27 states. It also includes Marketside-branded iceberg lettuce sold at Walmart locations in 15 states, according to the FDA.