Animal rights campaigners in the United States have begun a legal battle for the release of five killer whales.

The People for the Ethical Treatment of Animals (PETA) argue that the whales are being kept as slaves at the SeaWorld marine parks and should be set free under America's 13th Amendment.

"It's the first case in the history of the US that seeks to extend constitutional rights to living, breathing, feeling beings who happen not have been born human," PETA's legal counsel Jeff Kerr said.

Mr Kerr will be taking the case before a judge in California.

"The case is based upon the plain text of the 13th Amendment, which outlaws the condition of slavery, without any reference to any particular class of victims and without reference to persons," he said.

"Slavery does not depend on the species of the slave any more than it depends upon the gender, race, or religion of the slave."

Thomas Jefferson School of Law professor David Steinberg says he finds PETA's claim surprising.

"I never believed that I would hear an argument that human slaves were comparable to animals in a zoo but that is in essence PETA's argument," he said.

"The 13th Amendment was designed to abolish the abhorrent practice of slavery, of enslaving human beings without compensation, treating human beings as property.

"That is something entirely different from the question of whether a marine park like Sea World should or should not have orcas."

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But Mr Kerr said it was clear to him that the whales were suffering in captivity.

"By any reasonable definition, these five orcas are enslaved," he said.

"They were forcibly ripped from their homes and families, with whom they would have spent their entire life in nature, they're held captive in the equivalent of concrete bathtubs that cause them great stress and illness and make them aggressive.

"They're denied everything natural to them, they're forced to perform tricks for SeaWorld's profit and they've been turned into virtual breeding machines to churn out more performers for Sea World's tawdry shows.

"So these wild captured animals who are now forced to perform for fleeting human amusement, are by any reasonable definition slaves."

Professor Steinberg however says the 13th Amendment was never intended to apply to animals.

"It was obvious that slavery meant slavery of human beings," he said.

"The fact that it doesn't say people is because the term was unnecessary to the 13th Amendment, it would have been surplusage."

In a statement, SeaWorld said it had no higher priority than the welfare of the animals in its care.

Mr Kerr however is convinced that the days of marine parks for large mammals are numbered.

"This is going to happen whether it's in this lawsuit or next month or next year, the fundamental rights of animals to be free from human enslavement is going to be recognised," he said.

"It's just a question of when and we believe that the time is right, certainly we believe that the public is ready, we know the orcas are definitely ready and we believe that the law is in place.

"So we're anxious to have the case heard."