By Emma Jane Kirby BBC Geneva correspondent

The World Health Organization (WHO) has warned that terrorist groups could try to contaminate food supplies and has urged countries to strengthen their surveillance.

There has already been some examples of deliberate contamination of the food chain. It's only very few, but there has been some examples WHO report

In a special report, the leading UN health agency, said an attack using chemical or biological agents in food could lead to people dying or contracting serious illnesses like cancer.

The agency said it had not received any specific warnings of such an attack.

But it added that it viewed deliberate food contamination as "a real and current threat".

'Potential is there'

The 45-page booklet entitled Terrorist Threats to Food warns of the potential insertion of pesticides, viruses and parasites in food as "a way of deliberately harming civilian populations".

It cites examples of intentional food attacks of the past, including a salmonella outbreak in the US state of Oregon.

In that incident, more than 750 people became ill, after members of a cult contaminated restaurant salad bars.

The WHO director of food safety, Jurgen Schlundt, said the booklet was not designed to alarm but rather to try to alert governments to boost their surveillance and emergency response measures.

"There have already been some examples of deliberate contamination of the food chain. It's only very few, but there have been some examples. And we do know that the potential is there," he said.

"The way to try to deal with it is to strengthen some of the systems that we already have in place, but they need in some cases strengthening of certain areas."

Mr Schlundt added that natural outbreaks show the potential dangers of food-borne disease.

He said about 1.5 million people already die each year due to diarrhoea-related illnesses caught from eating contaminated food.

The WHO says if terrorists deliberately add harmful agents, many more people could be left suffering from acute long-term effects, including paralysis, foetal abnormalities and increased rates of chronic illnesses like cancer.