US law enforcement agencies have warned that the terror threat posed by home-grown extremists is greater than that posed by Isis.

A report issued by the US Department of Homeland Security this week in conjunction with the FBI, says that there have been 24 terror attacks by right wing extremists in the US since 2010.

Extremists have killed police officers, and threatened attacks on US government buildings says the report, which was seen by CNN.

"(Sovereign citizen) violence during 2015 will occur most frequently during routine law enforcement encounters at a suspect's home, during enforcement stops and at government offices," says the report.

It describes an incident in 2012, in which a father and son were involved in a shootout with police officers in Louisiana after being pulled over for a traffic violation. Three officers were killed in the confrontation. The men believed that the police had no authority over them, the report states.

"Law enforcement officers will remain the primary target of (sovereign citizen) violence over the next year due to their role in physically enforcing laws and regulations," the report adds.

A study last year by the University of Maryland's National Consortium for the Study of Terrorism and Responses to Terrorism found that most local and state law enforcement officers viewed the threat from domestic extremists as greater than that posed by Islamist radicals, such as those of Islamic State.

The report also describes a 2013 attack on three Transportation Security Administration employees in Los Angeles, in which one was killed, by a man who held radical anti-government views, and a shooting at a Las Vegas supermarket by anti-government extremists in 2014, in which two police officers and a bystander were killed.

According to the Southern Poverty Law Centre, the number of extremist organisations in the US has grown by 56 per cent since 2010, and now includes anti-government 'Patriot' groups, racist organisations, as well as far-left wing groups.

Next week, the White House is to hold an international summit on the threat posed by violent extremists, and critics have questioned why the threat posed by US domestic extremists is not on the agenda.