Scientists are trying to work out if the birds died of botulism, a rare but deadly illness caused by a toxic bacteria, or from pollution, the ministry said.

The ministry would not say what species of duck had been hit in the mass death, or how it had ruled out bird flu as a cause of death.

Although Mexico has had some cases of a low pathogenic strain of the flu, which presents no known risk to humans, it has so far been successful at keeping out the highly contagious and deadly H5N1 strain of the virus.

Migratory birds like ducks are seen as a possible cause of cross-border infection since flu was detected in birds in Russia, Turkey and Romania.

H5N1 is endemic in poultry in parts of Asia, where it has killed almost 70 people.

Experts fear the virus could mutate into a form which can be transmitted easily from person to person, sparking a pandemic in which millions of people could die.

Some environmental activists in Mexico say the virus could enter the country via a thriving trade, both illegal and legal, in exotic birds such as parrots.

Mexico prohibits the import of all birds and bird products from countries with confirmed outbreaks of the virus.