The animal rights group Peta has released shocking footage captured during a five-month undercover investigation which it says shows the violent abuse of sheep in shearing sheds across the US and Australia.

Warning: This story includes graphic video

Videos published last night reveal what campaigners said was evidence of widespread, criminal mistreatment of animals, punched, kicked and sometimes killed by workers.

Peta said its investigators had witnessed abuses in nearly every shed they visited while filming from October 2013 to February this year, in an industry "infested with violence".

It said the footage featured just a small selection of the cruelty observed at 14 sheds in Wyoming, Colorado and Nebraska in the US and 19 sheds in Australia - the latter country being responsible for an estimated 20 per cent of the world's wool exports.

Mimi Bekhechi, an associate director at Peta UK, said the videos were being sent to UK retailers in an appeal to them to stop selling wool products.

"Sheep are gentle prey animals who are petrified of even being held down, yet these sheep were punched in the face, kicked and stamped on and had their heads slammed into the floor by unsupervised, impatient shearers, causing them great distress, injury and even death," Bekhechi said.

"Peta is calling on shoppers around the world to reject cruelty to animals - and that means never buying wool."

In one incident from the US investigation, Peta said a shearer muttered "f***" over and over as he killed a sheep by twisting, severely bending and ultimately breaking her neck.

As he kicked the dying animal head-first down a chute, he apparently said: "I might have killed it." Peta said the shearer also admitted that he had previously injured another sheep by bending the animal's neck back, explaining: "I get angry."

In Australia, Peta claimed that workers for seven contractors were seen kicking, stomping or standing on animals' heads, necks and hind limbs. Eight contractors had workers who punched or struck sheep using their clippers.

Peta said workers regularly caused gaping wounds to the sheep with their shears, and then did not administer painkillers before using needles and thread to sew up the flesh.

The group also claimed that one worker was witnessed beating a lamb over the head using a hammer.

Investigating Peta's claims, NBC News was told by a trade group representing more than 27,000 Australian wool growers that it "categorically and unequivocally condemns the mistreatment of animals".

Michelle Lee, a spokesperson for Australian Wool Innovation, said: "Australian wool growers genuinely care for health and welfare of their animals so such alleged behaviour is very concerning."

A representative from the American Wool Council, the main US wool industry trade group, told NBC News the behaviour described by Peta was "unacceptable".

"We do not condone or support the actions of anyone that results in the abuse of sheep either intentionally or unintentionally," said Rita Samuelson.

"Rough handling of animals that might result in the injury of a sheep is an unacceptable manoeuvre during the shearing process or anytime when sheep are handled. … Kicking, throwing and poking the eyes of sheep are also unacceptable practices."

Peta US said it had asked state and local law-enforcement agencies to investigate and bring criminal charges against workers as appropriate.

Lt Gary Jackson from the Montrose County Sheriff's Office in Colorado confirmed to NBC that he had viewed the footage and prepared a report for the county's district attorney to review.

But he said: "I saw nothing in the video that is outside the norm for that industry."