New York mayor Bill de Blasio said it was "disrespectful" for some city police officers to turn their backs to him during two funerals for dead police officers.

The acts of protest were hurtful to the families of the two cops killed in an ambush last month, Mr de Blasio said in his first public remarks on the officers' protests.

The chasm between the police unions and the mayor has created the biggest crisis of his year-old tenure.

Police union leaders have said he contributed to an environment that allowed the officers' deaths by supporting protests following the police killings of unarmed black men in New York City and Ferguson, Missouri.

Mr de Blasio argued that the public rebuke was an offence to the city at large.

"Those individuals who took certain actions the last two weeks, they were disrespectful to the families involved. That's the bottom line," he said at a news conference at police headquarters.

"They were disrespectful to the families who lost their loved ones. I can't understand why anyone would do such a thing in the context like that."

Patrick Lynch, head of the city's rank-and-file police union, said after the deaths of Officers Rafael Ramos and Wenjian Liu that Mr de Blasio had "blood on his hands".

Thousands of officers turned their backs on the mayor when he delivered eulogies at Mr Ramos' funeral last month and again on Sunday at Mr Liu's funeral.

Police Commissioner William Bratton, who has steadfastly supported the mayor during the widening rift with the rank-and-file, also condemned the action, saying the officers involved "embarrassed themselves".

He called the protests a selfish act that dominated the news coverage and diverted attention from the two dead officers.