Environmental extremists and animal rights activists, including the British-based Stop Huntingdon Animal Cruelty, pose one of the most serious terrorism threats to the US, according to the FBI.
John Lewis, the FBI's deputy assistant director for counter terrorism, told a Senate committee that activists had claimed responsibility for more than 1,200 criminal incidents in the US since 1990, including arson and bombing attacks against animal research laboratories and the pharmaceutical and cosmetic industries.
"There is nothing else going on in this country over the last several years that is racking up the high number of violent crimes and terrorist actions," Mr Lewis said.
He singled out the Animal Liberation Front (ALF), the Earth Liberation Front (ELF) and Stop Huntingdon Animal Cruelty (Shac) as being of particular concern.
Mr Lewis said the FBI drew its conclusions after looking at all types of cases and comparing the animal rights groups with "rightwing extremists, KKK, anti-abortion groups and the like".
He said most animal rights and eco-extremists had so far refrained from violence targeting human life, but added that this could change. "We have seen an escalation in violent rhetoric and tactics," he told the Senate environment and public works committee.
"Attacks are also growing in frequency and size," he said, adding that it was plainly a matter of luck that nobody had been killed. "Once you set one of these fires they can go way out of control."
Shac was formed to close down the Cambridgeshire-based animal research firm Huntingdon Life Sciences, but seven of its US activists are due to go on trial next month on charges of vandalising com pany property and harassing lab employees and customers at the company's facility in East Millstone, New Jersey.
However, a spokesman for Shac in the UK laughed at the FBI allegations. "The FBI should remember they are not in the movies," he said. "If they think Shac is a terrorism organisation then they should get out a bit more. It's a funny terrorist organisation that has never killed a human, never harmed a human and does not want to bring down the government.
"If they think we are terrorists then they probably think Greenpeace and Friends of the Earth are terrorists as well."
Frank Lautenberg, a Democratic Senator from New Jersey who described himself as a "tree hugger", said: "The Department of Homeland Security spends over $30bn (£16.3bn) a year to protect the home front." After listing al-Qaida, Hamas and Hizbullah, he wanted to know who else the FBI considered terrorists: "Right to Life? Sierra Club? [America's largest environmental group]"
In the UK, the concerted campaigns by activists against pharmaceutical firms have been blamed, by the industry, on decreased spending on research and development. According to the Association of the British Pharmaceutical Industry R&D funding fell by £100m during 2002 and 2003.
"It is clear that the continuing threat posed by animal extremists is a contributory factor," said Vincent Lawton, the president of the trade association and an executive at the American drugs firm Merck.
The association said the number of incidents of damage to a company or personal property by animal rights extremists had increased by 18% in 2004 and abusive phone calls or texts increased almost three-fold. However, the number of demonstrations and visits to employees' homes had fallen.