The FBI has said it is monitoring a growing extremist threat from "sovereign citizen" movements, and is targeting their activities to prevent a repeat of the 1995 Oklahoma City bombing.

Followers of "sovereign citizen" ideologies reject all government authority, refusing to pay taxes, not recognizing US currency, and sometimes engaging in acts of violence, FBI officials told reporters on Monday.

"They could be lone wolves, too," said Stuart McArthur, deputy assistant director of the FBI's counter-terrorism division, who said it was the agency's mission to "deter and detect" potential acts of "catastrophic violence."

The worst act of domestic terrorism in US history was the Oklahoma City bombing on April 19, 1995, which claimed the lives of 168 people and left a federal office building in ruins.

Its main perpetrator, Timothy McVeigh, was sentenced to death and executed in 2001.

The FBI said the "sovereign citizen" militants are distinguishable from other ultra-right extremists by their use of homemade license plates, documents and diplomatic credentials, as well as the terminology they use.

"The ideology itself promotes violence and encourages violence," said special agent Casey Carty, who said the FBI had a duty to be proactive in dealing with the potential threat.

The number of convictions in "sovereign citizen" cases rose from 10 in 2009 to 18 in 2010 and another 18 last year.

"We want to focus on this particular threat," said McArthur, adding that evidence of the alarming movement is found "all over the country."

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