(Corrects paragraph 3 to read virus found at several markets, not wholesale suppliers)

HONG KONG, June 12 (Reuters) - Hong Kong officials scoured farms and wholesale markets on Thursday, searching for the source of an outbreak of the H5N1 bird flu virus that forced the culling of thousands of birds across the wealthy city.

Hong Kong ordered a cull of 3,500 poultry on Wednesday after discovering the H5N1 virus in four markets scattered across the city's northern and central areas, including in the commerce and retail hub of the main island.

Samples taken from local farms have so far tested negative. But officials found instances of the virus at several markets, which have since had their facilities cleaned and disinfected.

"We are working on finding the origin of this virus but have had no result yet," said Sally Hong, a spokeswoman for the Agriculture, Fisheries and Conservation Department.

"We're also monitoring the health of poultry traders and workers to see if they have any symptoms of flu."

Since the H5N1 virus resurfaced in Asia in 2003, it has killed 241 people in a dozen countries, according to the World Health Organisation.

Experts fear the constantly mutating virus could change into a form easily transmitted from person to person. (Reporting by Amrita Sheokand; Editing by Nick Macfie)

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