The World Health Organization on Wednesday said a batch of cough and cold syrup sold in Cameroon under the brand name Naturcold contained extremely high levels of a toxic ingredient, the latest in a series of recent warnings about contaminated cough syrups.

The packaging label on the syrup showed it was manufactured by a company called Fraken International (England), but the UK health regulator said no such company exists in the country, the WHO said.

"Enquiries are still underway to determine the origin of the product," WHO said. A spokesperson told Reuters that the syrup may be on sale in other countries as well as Cameroon, which prompted its global alert calling for more surveillance.

In 2022, more than 300 children - mainly aged under five - in Gambia, Indonesia and Uzbekistan died of acute kidney injury, in deaths associated with similar products made by other manufacturers. The WHO has said the threat is ongoing.

The alert in Cameroon follows the country's health regulator saying in April that it was investigating the deaths of six children linked to Naturcold. The WHO had told Reuters it was supporting the authorities there.

The acceptable limit for diethylene glycol, the contaminant found in the syrup, is no more than 0.1%, according to the WHO, but the Naturcold batch had syrups that contained as much as 28.6% of the substance.

Unscrupulous actors sometimes substitute propylene glycol, an ingredient used in the syrups, with cheaper but toxic alternatives like ethylene glycol and diethylene glycol, several pharmaceutical manufacturing experts have told Reuters.

The contaminants can cause abdominal pain, vomiting, diarrhoea, an altered mental state and acute kidney injury, among other symptoms, which may eventually lead to death, they said.