Authorities on Tanzania's semi-autonomous Zanzibar archipelago said they were investigating the mysterious deaths of chickens for a possible outbreak of bird flu.

The multiple deaths of the birds at a poultry farm on the main island of Unguja prompted the probe, which comes amid heightened fears of the further spread of the deadly H5N1 strain of the virus in Africa, they said on Tuesday.

"This is fresh bad news to me, we have to investigate to identify the possible cause of the chickens' death," said Kassim Gharib, the island's director of livestock development.

"We have to be alert on any problem connected to poultry deaths because bird flu is now a threat to the world," he told AFP. "We must remain on high alert in all areas and continue to seriously step up prevention and control work."

Zanzibar's state-owned Zanzibar Leo daily reported Tuesday that at least 30 chickens at a farm in Kinyasini, 28 kilometers (17 miles) from the island's capital, had died over the past week of an unknown illness that could be bird flu.

Zanzibari authorities banned the importation of poultry and poultry products two weeks ago after a trader was arrested smuggling 40 chickens into Zanzibar from Brazil through Tanzania's commercial capital Dar es Salaam.

Dozens of countries have now been hit by the H5N1 strain that originated in Asia and has spread west to Europe and Africa, killing more than 100 people since 2003.

In Africa, it has so far been found in Burkina Faso, Cameroon, Egypt, Nigeria and Niger.