ISLAMABAD, Aug 21 (Reuters) - Pakistani authorities have detected an outbreak of the H5N1 strain of bird flu at a poultry farm in a northwestern town and have culled more than 35,000 birds, a government official said on Tuesday.

Several outbreaks of H5N1 strain of avian influenza have been found in birds this year in Pakistan, where the virus first appeared in early 2006. Pakistan has had no human cases.

The new outbreak was found in late July at a big farm in Mansehra town, 80 km (50 miles) north of the capital, Islamabad.

"Samples were tested after reports of the death of about 14,000 chickens," said Food and Agriculture Ministry official, Rafiq-ul-Hassan Usmani.

"The remaining 35,000 birds were culled after the samples tested positive," he said.