A US federal appeals court upheld California's ban on the sale of foie gras Friday, 13 months after the block on the controversial delicacy came into force.

The Ninth Circuit Court of Appeals rejected claims that the ban interferes with free trade, made in a lawsuit filed in Los Angeles by a group of Canadian and US foie gras producers.

California lawmakers agreed the ban in 2004, but gave the western US state's only foie gras producer seven-and-a-half years to comply before it came into effect on July 1 last year.

Restaurants serving the gourmet item -- made by force-feeding ducks or geese, a practice some animal rights campaigners regard as cruel -- can be fined up to $1,000.

The 27-page opinion of a three-judge panel, issued Friday, includes a detailed description of how foie gras is produced, by force-feeding ducks so they store extra fat in their livers.

The producers who brought the lawsuit, including New York state-based Hudson Valley Foie Gras, the biggest US supplier of the delicacy, are expected to appeal Friday's ruling again.

In the run-up to the July 1, 2012 ban, some of the Golden State's top chefs, calling themselves the Coalition for Humane and Ethical Farming Standards (CHEFS), redoubled efforts to persuade lawmakers to overturn the ban.

They staged a series of foie gras-rich evenings to raise money for the cause. But John Burton, the former lawmaker who drafted the legislation, likened foie gras production to outlawed practices such as waterboarding or female genital mutilation.

"I'd like to sit all 100 of them down and have duck and goose fat -- better yet, dry oatmeal -- shoved down their throats over and over and over again," he told the San Francisco Chronicle.

[Image via Agence France-Presse]