Several outbreaks of the H5N1 strain have been found in birds in Pakistan over the past year and the first human case was confirmed late last year.

In December, the World Health Organisation said it had established a single case of human infection of bird flu in a sick family in Pakistan but there was no apparent risk of it spreading further. The man died.

"Laboratory tests have confirmed the presence of the virus in a poultry farm in Karachi, and we are now monitoring the workers on the farm as well as another one adjacent to it," said an official of the Sindh provincial government.

Karachi is the capital of Sindh.

"But there are no signs of any human infection," the official said.

Kamal Akhtar Siddiqui, general secretary of the Karachi Wholesale Poultry Association, said about 5,500 chickens at the farm had died because of the virus while 500 that survived had been culled.