Image caption Experts say some victims do not report hate crimes for fear of reprisal

Nearly two in three hate crimes in the US are not reported to police as victims doubt police can or will help, justice department statistics showed.

From 2003-06, 46% of hate crimes were reported, but police were notified only 35% of the time from 2007-11.

The proportion of victims who did not report crimes fearing police would not help rose to 24% from 14%, the Bureau of Justice Statistics said.

Experts said the data comes amid a rise in the number of hate groups.

The study found there was an average of 259,700 hate crime incidents on people over the age of 12 between 2007-11.

What's surprising about this is that knowledge of hate crimes is far more prevalent across the country than ever Chuck Wexler, Police Executive Research Forum

The percentage of crimes motivated by religious bias more than doubled between 2003-06, but there was a decline in attacks motivated by racial hatred, it said.

Between 2007-11, whites, blacks and Hispanics reported similar rates of violent hate crime victimisation.

"It's shocking to see that much of an increase in the feeling of futility that hate crime victims are apparently experiencing," Jason Marsden, executive director of the Matthew Shepard Foundation, told the Associated Press.

The group was created by the parents of a gay college student killed in an attack in 1998 that police said was partly motivated by his sexual orientation.

Jim Bueermann, president of the Police Foundation, a research organisation, said hate groups were becoming more violent, and victims may be afraid of reporting attacks for fear of reprisal.

The civil rights group Southern Poverty Law Center said that for each of the last three years it has identified more than 1,000 organised hate groups, up from 600 to 700 such groups between 2000-02.

Experts say the decline in reporting comes despite rising awareness of hate crimes.

"What's surprising about this is that knowledge of hate crimes is far more prevalent across the country than it ever has been at any time in our history," said Chuck Wexler, executive director of the Police Executive Research Forum.

In Congress, hate crimes are defined as criminal offences motivated by a bias against race, religion, disability, ethnic origin or sexual orientation.

Thursday's findings come mainly from data collected on hate crimes since 2003 for the National Crime Victimization Survey.

It is based on annual Census Bureau interviews with a large representative number of people about their experiences with crime.