Experts are still trying to determine the source of an outbreak of bird flu at a Suffolk farm as nearly 160,000 turkeys are culled.
The government says the priority is to contain and eradicate the disease, identified as the H5N1 strain, at the Bernard Matthews site near Lowestoft.
Strict controls are in place around the site, where turkeys are being gassed.
Officials say the risk posed to humans is "negligible", but there are concerns poultry sales will be hit.
Experts are trying to find the source of the outbreak of the disease, identified as the "highly pathogenic" Asian strain of the H5N1 bird flu virus.
Tests could take weeks and it is possible the exact cause will never be known.
Meanwhile, Rural Affairs Minister Ben Bradshaw told the BBC he expected the cull of turkeys at the farm to be finished later.
He said more than 120,000 turkeys had been culled in just two days. Carcasses are being incinerated at a rendering plant in Cheddleton, Staffordshire.
The government's chief scientific adviser, Professor Sir David King, told the BBC the virus was identical to a case in Hungary in January.
"There could be a direct contact between a Hungarian farm and the Bernard Matthews farm, by which I mean a person travelling from one to the other," he said.
"Or more likely it is wild birds travelling across to Britain from that area."
Speaking after a meeting of the government's emergency planning committee Cobra, Mr Bradshaw added: "It's not really terribly helpful to speculate at this stage what might have caused it.
"What's much more important is that we satisfy ourselves that the outbreak has been eradicated and contained."
A biosecurity zone has been set up around the farm at Holton, south-west of Lowestoft. A 3km (1.9 miles) protection zone and a 10km (6.2 miles) surveillance zone remain in place.
Poultry owners in a wider restricted zone have been told to keep their flocks isolated from wild birds
But BBC reporters discovered some chickens left outside just a mile from the farm.
Mr Bradshaw said anyone breaching current instructions should be reported to the police, Defra or trading standards and officials would be scouring the area to make sure biosecurity was being adhered to.
The authorities have denied their response was slow saying restrictions were immediately put on the farm, once it became clear birds were dying in large numbers.
Japan and Russia have temporarily suspended imports of UK birds.
And Charles Bourns, chairman of the National Farmers Union's poultry board, said farmers were worried about the potential impact on sales and urged shoppers to keep eating British poultry.
Health experts have been trying to quell fears the outbreak could spread to humans.
But the government is preparing for the small chance the disease could mutate into a form which causes a flu pandemic.
Health Secretary Patricia Hewitt said: "It is a very remote risk but if it did happen it could be very serious indeed."
The avian flu virus has so far killed 164 people worldwide - mainly in south-east Asia - since 2003. All those who contracted the virus had come into close contact with infected birds.
The virus has so far been detected in only one shed out of 22 at the site. About 100 of the 2,000 staff at the Holton farm have been given the Tamiflu anti-viral drug.
A spokesman for Bernard Matthews said: "None of the affected birds entered the food chain and there is no risk to consumers.
"Bernard Matthews continues to work very closely with Defra and other industry bodies. All proper procedures were followed and we responded accordingly in good time."
He added that the company had reassured staff and said there were no jobs at risk.
For more information call the Defra Helpline on 08459 33 55 77
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Scene of outbreak
All poultry to be culled
Visitors disinfected and restricted access
3km Protection Zone
Poultry kept indoors and tested
10km Surveillance Zone
No movement of poultry to or from area except for slaughter
Trains carrying live poultry are prevented from stopping in the protection zone
Bird fairs and markets banned
Increased surveillance of wetland areas
Domestic birds not to share water used by wild birds
Footpath restrictions likely only on free-range farms
People in towns not affected unless they keep poultry.
Isolation of poultry from wild birds
Poultry movements to be licensed