US police departments and union leaders are warning the rank and file to wear bulletproof vests and avoid making inflammatory posts on social media in the days after a man ambushed two officers and shot them to death inside their patrol car.

New York City mayor Bill de Blasio called for a temporary halt to protests over police conduct as he faced a widening rift with a force preparing Christmas time burials for two of its own and decrying the demonstrations as a factor in the officers’ cold-blooded executions.

The gunman who murdered two New York City police officers in their squad car had a long criminal record, a hatred of police and the government and an apparent history of mental instability that included a suicide attempt a year ago, it has emerged.

US president Barack Obama has offered full support and federal assistance to the New York Police Department after two of its officers were shot dead.

New York City’s police commissioner says the gunman who ambushed and killed two officers had made online posts that were “very anti-police”.