DHAKA, March 30 (Reuters) - Bird flu has spread to five more farms in central and northern districts, Bangladesh said on Friday.

"The avian flu has now spread to 16 farms in five districts so far. But no humans have been confirmed infected," an official of the fisheries and livestock ministry said.

Bangladesh has culled about 60,000 birds since confirming the outbreak of the H5N1 flu virus simultaneously on six farms near the capital, Dhaka, on March 22.

All the culled birds belonged to the 16 affected farms. So far, 30 workers in the five newly affected farms were being monitored after they were given health checks.

Earlier, 100 poultry workers were released after no H5N1 infection was detected by a local laboratory, which tested their blood and other samples days after the initial outbreak was detected.

Movement of chickens has been banned outside a 10 sq km (3.9 sq miles) area around the affected farms, officials said.

Livestock officials said after the outbreak of the virus they had inspected about 10,000 farms containing nearly 1.1 million birds.