HOLTON, England - Nearly 160,000 turkeys were being culled in Britain Sunday after the country's first major outbreak of the potentially lethal Asian strain of the H5N1 bird flu virus.

The slaughter of birds at a huge factory farm in Holton, Suffolk, eastern England, started late Saturday and was expected to take about two days, an environment ministry spokeswoman said.

After the poultry have been gassed, their carcasses will be transported in sealed, leak-proof lorries to a plant around 200 miles (320 kilometres) away in Staffordshire, north-west England, where they will be destroyed.

Preliminary tests confirmed the presence of the H5 strain of bird flu late Friday. H5N1 was confirmed early Saturday and the virulent Asian strain - similar to that found in Hungary last month - was confirmed later in the day.

Scientists fear that H5N1 -- which has killed more than 160 people globally since 2003, mostly in South-east Asia -- could mutate into a form easily transmissible between humans, sparking a pandemic that could kill millions.

But British government scientists said the outbreak was being contained and posed no immediate danger to human health. Farmers and public health officials said well-cooked meat was still safe to eat.

Despite stringent biosecurity measures now in place, newspaper reports claimed bosses at the Bernard Matthews Holton site and the environment ministry did not act quickly enough after the first birds died.

The Sunday Telegraph claimed 71 birds at the farm in Holton, eastern England, died in unexplained circumstances last Tuesday but the matter was not reported to vets until Thursday, when more than 1,000 others had perished.

It also alleged that some of the farm's 1,000-strong workforce were allowed to walk around the site and its 22 turkey sheds for two days before their movements were restricted.

But an environment ministry spokeswoman stressed that the farm and vets had "acted responsibly".

"Among eight-week-old birds in a plant this size, it's not unusual for that number to die," she said, adding that a private vet had been called on Tuesday before an escalation of deaths on Thursday led to state vets being notified.

"There will be an investigation of what took place at the plant."

As a precaution, farm workers and those involved in the cull have been given anti-viral drugs while vehicles using a neighbouring meat factory were disinfected.

A three-kilometre (1.8-mile) protection zone and 10-kilometre surveillance zone was put in place around the site. All poultry were ordered to be isolated from wild birds and any movement had to be licensed.

The environment ministry widened the restriction zone late Saturday to cover an area of 2,090 square kilometres (810 square miles).

"Targeted surveillance" is also in place across large swathes of England, Wales and Scotland for H5N1 and other avian influenza viruses in wild birds.

Other measures introduced include the withdrawal of licences for bird shows or gatherings and a ban on pigeon racing.

It is not yet clear how the animals were infected with H5N1, although Britain's deputy chief veterinary officer said he believed it had been carried by a wild bird.

The outbreak was found in only one of the 22 turkey sheds on the farm, which is in the heart of England's chicken and turkey-rearing region in eastern England.

Bernard Matthews told customers there were strong biosecurity measures in place and said none of the affected birds had entered the food chain.

Last month's Hungarian outbreak of Asian strain H5N1 was detected among geese and was the first within the European Union since mid-2006.

News of Britain's outbreak prompted the Dutch agriculture ministry to announce extra protective measures, Norway to impose an export ban on British poultry and France to evaluate the risk to its flocks. - AFP/ir