ISLAMABAD (Reuters) - Health authorities in Pakistan aredoing tests on 12 workers from a farm where the deadly H5N1bird flu virus was detected in poultry, a Health Ministryspokesman said on Tuesday.

Authorities confirmed on Monday an outbreak of the H5N1strain of bird flu at a poultry farm on the outskirts ofKarachi, the second case in four days in the country's biggestcity.

Twelve workers from the farm had been isolated and werebeing tested by a team from the World Health Organisation(WHO), the ministry spokesman said.

"They have been isolated and now a WHO team is there and isgoing to take another sample. Preliminary samples were takenand found negative but the WHO wants to confirm it," said thespokesman, Orya Maqbool Jan Abbasi.

The H5N1 virus was first detected in Pakistan in early2006. Pakistan's first human case, which resulted in a death,was confirmed in December.

Authorities have found several outbreaks in poultry andother birds in North West Frontier Province, Islamabad and mostrecently Karachi, where the new outbreak was found on a farm300 meters (yards) from where the virus was detected last week.

Tests were also being done on samples taken from poultry inPunjab province but no outbreak had been confirmed, Abbasisaid.

"There have been a few incidents in which birds have diedbut it could be due to other conditions, maybe the weather andpoor heating. It's the first time in 50 or 60 years that suchlow temperatures have been faced by the Punjab people," hesaid.

A woman had been tested for the virus in Peshawar, in NorthWest Frontier Province, but found to be negative, he said.

In December, the World Health Organisation said it hadestablished a single case of human infection of bird flu in asick family in North West Frontier Province, but there was noapparent risk of its spreading further. The man died.

Meanwhile, Afghanistan has banned the import of chickensand poultry form Pakistan because of the bird flu outbreak, anAfghan border security official said.

"We have very strict orders from Kabul," Afghan borderpolice official Akhtar Jan told Reuters in the town of SpinBoldak, one of two main official border-crossing points betweenthe neighbors.

"We're not allowing any vehicle from Pakistan carryingchickens, chicken meat or eggs," he said.

Afghanistan first detected the H5N1 virus in poultry inearly 2006.

(Reporting by Robert Birsel and Saeed Ali Achakzai; Editingby Katie Nguyen)