ISLAMABAD, May 23 (Reuters) - Pakistani authorities have discovered the deadly H5N1 strain of bird flu at three poultry farms on the outskirts of Islamabad and culled nearly 10,500 chickens, a government official said on Wednesday.

Pakistan first detected the H5N1 strain of the virus in early 2006 in North West Frontier Province and about 40,000 birds were culled. Several outbreaks have been found in chickens this year but Pakistan has had no human cases.

Samples were collected on Monday after about 2,500 birds died on the three farms last week. The samples tested positive for the H5N1 strain, said Food and Agriculture Ministry official Rafiq-ul-Hassan Usmani.

"We took action on Tuesday and killed the remaining birds on the three farms which had some 13,000 chickens before the virus struck," Usmani said.

Poultry has been culled on several small farms this year after the virus was found and authorities briefly shut Islamabad Zoo in February after four peacocks and a goose died of the H5N1 strain.

Usmani said authorities were investigating the new cases to see if the farm owners ignored instructions to vaccinate their birds.

"We are keeping a close watch and there is no cause of concern," he said.

The virus has killed more than 180 people, most of them in Southeast Asia, since it re-emerged in 2003. Experts fear it could mutate into a form that could jump easily between people and cause a pandemic.