Hong Kong - The spectre of workers in protective suits slaughtering chickens by the tens of thousands returned to haunt Hong Kong Wednesday as a worrying new outbreak of bird flu was confronted.

More than 60,000 birds were culled at the farm where the city's first outbreak in six years was detected and thousands more were killed at a nearby farm and a wholesale market.

And while Hong Kong's health secretary insisted the outbreak was under control, scientists were puzzling over how chickens vaccinated against the virus had fallen prey to bird flu.

The new scare in the city that saw the first human deaths from bird flu in modern times in 1997 began when it was announced Tuesday that 200 chickens had died of the H5 virus at one farm.

Workers in protective suits and masks killed 3,000 chickens Tuesday evening and returned to cull the remaining 60,000 birds at the farm in the Yuen Long district Wednesday morning.

Another 10,000 chickens were due to be culled at a nearby farm and a wholesale market in the area, officials said, saying they hoped to complete the slaughter Wednesday.

Hong Kong health secretary York Chow said Wednesday morning the outbreak was "under control." Inspectors had visited all local farms in the area and found no further signs of infection, he said.

Chow said government scientists were examining why the H5 virus had apparently been able to infect birds vaccinated against bird flu and would consider whether alternative vaccines were needed.

Currently, Hong Kong uses a Dutch-made vaccine to protect chickens against the virus whereas some experts say the city should switch to a different vaccine used in mainland China.

"We have to be rely on scientific experts' opinion on which vaccine we should use and how we should use it," Chow told reporters.

Tests were meanwhile continuing Wednesday to establish whether the birds infected in the new outbreak were infected by the H5N1 strain of bird flu that more easily jumps the species barrier to humans.

Six people died and 12 others were infected in an outbreak of bird flu in Hong Kong in 1997 that led to the culling of 1.2 million birds. Millions more birds were slaughtered in outbreaks in 2001 and 2002.

Hong Kong subsequently implemented strict controls on markets and chicken imports and escaped any human infections when bird flu swept the region in 2006.

However, the new outbreak raises the possibility that the virus has mutated to develop immunity to the vaccine currently being used on chickens.

Experts have repeatedly warned that the H5N1 strain of bird flu threatens a global pandemic if it mutates into a form that is more easily transmitted between humans. (dpa)