Attacks perpetrated by lone actors are the biggest terrorism threat to July 4th festivities in New York City and elsewhere, federal authorities said in a threat assessment obtained by CNN.

Intelligence bulletins sent out ahead of the holiday warn of copycat attacks and homegrown extremists targeting July 4 celebrations in City and San Francisco.

As cities across America prepare for massive July 4 gatherings, federal authorities have sounded the alarm over potential security threats from lone-wolf attackers and unauthorised drone activity.