DHAKA, April 11 (Reuters) - Bird flu has spread to the southern region of Bangladesh, despite persistent efforts by veterinary and health personnel to contain it, a senior official of the fisheries and livestock ministry said.

"The avian virus has been detected at a farm in Noakhali district 200 km (125 miles) south of the capital Dhaka," said the official, Mohammad Abdul Motalib of the livestock department.

He said the H5N1 virus spread despite a struggle by hundreds of veterinary and health officials to hold it in check.

Movement of chickens had been banned outside a 10 sq km (3.9 sq miles) area around affected farms.

More than 75,000 chickens have been culled so far from 28 farms since the outbreak of avian flu was confirmed simultaneously in six farms at Savar near Dhaka on March 22.

Some 550 workers at the infected farms have been given a local version of the Tamiflu anti-viral drug as a precaution, Health Ministry officials said.

No humans have tested positive for the disease in densely populated Bangladesh.

The government says it has sufficient Oseflu, a local version of Tamiflu, produced and marketed by a local firm since last year.

The virus is known to have infected nearly 300 people in 12 countries since 2003, killing more than half of them.

Human cases of bird flu have generally been linked to contact with infected poultry. Health experts fear the virus may mutate into a form that passes easily from human to human, causing a pandemic that could affect millions.

Bangladesh has 125,000 small and large poultry firms producing 250 million broilers and 6 billion eggs annually.

About four million Bangladeshis are directly or indirectly associated with poultry farming.