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ROME, April 28 (Reuters) - The United Nations' food agency said on Tuesday its was mobilising its animal health experts and sending some to Mexico to check if the new strain of flu virus widely described as swine flu is really directly linked to pigs.

"At present, transmission seems to be occurring solely from humans to humans," said the Food and Agriculture Organisation.

"So far evidence that the new strain of influenza A virus has entered the human population directly from pigs has not been established," the Rome-based U.N. body said in a statement.

The FAO's Chief Veterinary Officer Joseph Domenech said there was "no evidence of a threat to the food chain; at this stage it is a human crisis and not an animal crisis".

The flu outbreak has killed as many as 149 people in Mexico and the World Health Organisation has indicated a significantly increased risk of pandemic, though so far the virus has not caused any known deaths outside of Mexico.

Despite advice that the virus is not caught from eating pork products, several countries have banned pork imports from the United States, where 50 infected people have been found.

But animal health authorities such as the Paris-based World Organisation for Animal Health (known as the OIE) say it should not be termed "swine flu" as it has avian and human components and no pig has so far been found ill with the disease.

The European Commission says it has no plans to restrict trade in pork as the flu has nothing to do with the food chain.

The FAO said it was sending FAO-OIE experts to Mexico this week to help the government "assess the epidemiologic situation in the pig production sector", while FAO staff around the world were on "full alert" to report any flu-like illness in pigs.

The Rome-based FAO urged governments and the international community to step up disease surveillance in swine. (Writing by Stephen Brown; Editing by Charles Dick)

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