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.

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.

A veteran Florida police officer was shot and killed while on duty early yesterday by a man described as transient, according to law enforcement officials, who gave no motive for the suspect now charged with murder.

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”.