WASHINGTON (Reuters) - Anti-government extremists opposed to taxes and regulations pose a growing threat to local law enforcement officers in the United States, the FBI warned on Monday.

These extremists, sometimes known as “sovereign citizens,” believe they can live outside any type of government authority, FBI agents said at a news conference.

The extremists may refuse to pay taxes, defy government environmental regulations and believe the United States went bankrupt by going off the gold standard.

Routine encounters with police can turn violent “at the drop of a hat,” said Stuart McArthur, deputy assistant director in the FBI’s counterterrorism division.

“We thought it was important to increase the visibility of the threat with state and local law enforcement,” he said.

In May 2010, two West Memphis, Arkansas, police officers were shot and killed in an argument that developed after they pulled over a “sovereign citizen” in traffic.

Last year, an extremist in Texas opened fire on a police officer during a traffic stop. The officer was not hit.

Legal convictions of such extremists, mostly for white-collar crimes such as fraud, have increased from 10 in 2009 to 18 each in 2010 and 2011, FBI agents said.

“We are being inundated right now with requests for training from state and local law enforcement on sovereign-related matters,” said Casey Carty, an FBI supervisory special agent.

FBI agents said they do not have a tally of people who consider themselves “sovereign citizens.”

J.J. MacNab, a former tax and insurance expert who is an analyst covering the sovereign movement, has estimated that it has about 100,000 members.

Sovereign members often express particular outrage at tax collection, putting Internal Revenue Service employees at risk.