Police have stepped up patrols of an elementary school in Minnesota after it received threats in the wake of accusations that it was using public funds to teach Islam.

The threats came after a local columnist wrote that the Tarek ibn Ziyad Academy, a suburban Minneapolis charter school run by an Islamic charity, appeared to be violating a ban on teaching religion in public schools.

Charter schools are public schools run by private organizations with public funds.

While many have been started by religious groups, they are bound to US rules that public schools must accommodate the religious needs of their students but are not allowed to promote religious views or lead prayer services.

The brewing controversy came to head in recent days when a substitute teacher said she saw students "corralled" into involuntary prayer services, and a local television station criticized the school for failing to fly a US flag.

The story got picked up on anti-Muslim websites and the school started getting threatening calls and e-mails, including threats to burn it down and "destroy" its students and leaders.

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