The Obama administration has issued a chilling warning to U.S. police forces of the threat of a rise in violent right-wing extremist groups fueled by economic recession, the return of disgruntled army veterans from Iraq and Afghanistan and hostility towards election of the first black president.

The internal report, which was not meant for publication, was drawn up by the department of homeland security, set up after the 9/11 attacks to co-ordinate internal security. A leaked copy said: "The economic downturn and the election of the first African-American president present unique drivers for right-wing radicalization and recruitment."

It added that the threat posed by lone wolves and small terrorist cells is more pronounced than in past years.

The appearance of the report generated criticism from Republicans and military veterans' groups, who resented the implication that returning troops presented a danger. One of the biggest veterans' groups, the American Legion, described it as "unfair‚" "incomplete‚" and "politically-motivated".

The nine-page report, marked For Official Use Only, was drawn up by the Homeland Security Department's extremism and radicalization branch. A leaked copy initially appeared on conservative websites and was then picked up by the U.S. media.