The FBI has issued a warning to all police departments in the US that an anarchist group may be planning to stage ambushes of police officers in various locations on Halloween.

It identified the group as the National Liberation Militia which has encouraged supporters to create disturbances to attract the police and then attack them, according to the New York Post and Fox News.

The militia has proposed a "Halloween Revolt" to attack the police. It recommended to supporters to wear typical Halloween masks and use weapons such as bricks, bottles and firearms.

The NYPD Intelligence Bureau said it is monitoring the threat.

The threat report comes at a time of tension between many police departments and the neighbourhoods they cover.

Patrick Lynch, head of the NYPD union, has called to boycott Quentin Tarantino's films after the latter participated in an anti-police rally in Greenwich Village.

"It's no surprise that someone who makes a living glorifying crime and violence is a cop-hater, too," Lynch said Sunday. "The police officers that Quentin Tarantino calls 'murderers' aren't living in one of his depraved big-screen fantasies — they're risking and sometimes sacrificing their lives to protect communities from real crime and mayhem."

The rally was held days after New York City cop Randolph Holder was shot dead while pursuing a bicycle thief in East Harlem.

"As part of the continuous dialogue with our law enforcement partners, the FBI routinely shares information about potential threats to better enable law enforcement to protect the communities they serve. We urge the public to remain vigilant and report suspicious activity to law enforcement," the FBI said Tuesday.

The FBI said 96 law enforcement officers were killed in line-of-duty incidents in 2014. Of these, 51 law enforcement officers died as a result of felonious acts, and 45 officers died in accidents.

It added that there were 48,315 officers who were victims of line-of-duty assaults.