American Jewish groups are fighting a proposed ban of Sharia law in the state of Oklahoma.

A coalition of organisations that include the Council on American-Islamic Relations and the American Jewish Committee (AJC) are backing a legal appeal against a measure that would prevent Islamic law from being used in the state's courts.

The AJC filed a brief last week with the US Court of Appeals arguing that the ban is an attack on religious freedom.

The AJC said the provision - called the "Save our State law" by its supporters and adopted after a referendum which delivered a 70 per cent "yes" vote last November - is unconstitutional, violating a clause in the First Amendment which bars a state preference for one religion over another.

"In a nation that treasures religious freedom and whose constitution forbids government to have favoured or disfavoured faiths, the Oklahoma provision cannot stand," wrote AJC Associate General counsel Marc Stern.

The Anti-Defamation League and the Union of Reform Judaism supported the brief, as did the Centre for Islamic Pluralism and the Baptist Joint Committee for Religious Liberty.

A judge issued an injunction in November putting the law on hold while the challenge is being heard.

Stephen Schwartz, who converted to Islam as an adult and is the executive director of the Centre for Islamic Pluralism, said both Jews and moderate Muslims are interested in protecting religious freedom. "This is not American, to pass a law in any state that's against the practising of a religion," he said.

He said American Jewish groups often work with mainstream Islamic organisations. "The AJC has organised three trips for moderate Muslims to visit Israel," Mr Schwartz said.

In the original lawsuit against the Oklahoma measure, Muneer Awad of the Council on American Islamic Relations said the law would infringe the rights of Oklahomans to wear religious head scarves, choose Islamic marriage contracts or to be buried according to Islamic practices.

He said the purpose of the law was made clear by Oklahoma Representative Rex Duncan, who said on television that "America was founded on Judeo-Christian principles".