Farmers fear animal rights extremists may be stepping up action against them after the Animal Liberation Front claimed responsibility for a petrol bomb attack.

The ALF claimed on a website that it was behind two petrol bombs which caused half a million pounds worth of damage to Field Farm, Appleton.

Two tractors, containers and 200 hay bales were destroyed in the attack last month. ALF has vowed to continue targeting businesses and farms it believes are 'abusing' animals.

Andrew Forsyth, local National Farmers' Union group secretary, said: "It is a worrying development and a different step from the normal intimidation and harassment.

"Taking direct action of any sort that endangers human life, property, and the animals they are trying to protect, is a concern because it is unlawful and a very dangerous game to play."

The NFU will be speaking to farmers about security, though Mr Forsyth said there were no specific plans to tackle the threats. He said: "It is too early to say if this is a change in tactics. It is all very new.

"Farmers regard themselves as very good custodians of livestock who look after animals in a humane way and do what they do as best as possible.

"Our advice to farmers is to be alert and keep an eye on strange people and unexpected vehicles - and anybody asking questions."

He added it was too early to say if the Appleton attack was a one-off, or the start of a campaign.

Neil Rowe, manager at Manor Farm, in Marcham, an organic farm where livestock make their own way to the dairy barn and milk themselves using high-tech computer-driven milking machines, said: "We operate above the welfare standards and are very happy with that.

"I think you are always going to have lunatics on the fringe of society with their own way of thinking.

"We will wait and see if it is a one off, or if two or three more happen."

Police spokesman Victoria Bartlett said: "This type of criminality is very rare and we would like to reassure farm owners that we do take incidents of this nature extremely seriously and will be investigating fully."

Anyone with information on the attack should call police on 08458 505505 or Crimestoppers on 0800 555111.