HOLTON (Reuters) - Britain gassed tens of thousands of turkeys and extended restrictions on the movement of poultry on Sunday to try to prevent the spread of deadly bird flu from a farm in eastern England.

The discovery of the highly pathogenic H5N1 strain of avian flu on a farm run by Europe's largest turkey producer surprised experts and raised questions about how the virus had been introduced into a sealed shed.

However, the Department for the Environment, Food and Rural Affairs (Defra) expressed optimism on Sunday that the virus had been confined to the Bernard Matthews farm near Lowestoft in the county of Suffolk.

"Obviously, though, we need to be very vigilant in the coming days," a spokeswoman said.

The slaughter of 159,000 birds at the farm, which started late on Saturday, was expected to take another two days. The dead birds were being transported from the farm in sealed trucks to be incinerated.

About 2,500 turkeys died in the initial outbreak of the virus, which appears to have been confined to one of 22 sheds at the farm.

Restrictions on the movement of poultry were extended overnight to a 2,000-sq-km (775-sq-mile) area around the farm.

Defra said the virus was the same pathogenic Asian strain found last month in Hungary where an outbreak among geese on a farm prompted the slaughter of thousands of birds.

That outbreak followed a relative lull in cases of H5N1 among European poultry since hundreds of turkeys died at a farm in eastern France about a year ago.

The strain tends to be transmitted to poultry by infected migrating wildfowl.

It has killed at least 165 people worldwide since 2003, most of them in Asia, and more than 200 million birds have died from it, or been killed to prevent its spread.

However. it has not yet fulfilled scientists' worst fears by mutating into a form that could be easily transmitted between humans and possibly cause a global pandemic.