Two New York City police officers have been killed by a lone gunman as they sat in their parked patrol car in broad daylight.

Both officers were shot in the head in the Bedford-Stuyvesant neighbourhood of Brooklyn by the suspect, who has been named as Ismaaiyl Brinsley.

Police Commissioner William Bratton identified the two officers as Liu Wenjin (32) and Raphael Ramos (40).

He said they were shot and killed "with no warning, no provocation. They were quite simply assassinated."

Mr Liu, a seven-year veteran who got married two months ago, and Mr Ramos, father to a 13-year-old son, were "ambushed and murdered", officials said.

Mr Bratton added that Brinsley had published posts on Instagram that were "very anti-police".

Authorities have not revealed the contents of the messages.

But two officials told the Associated Press that Brinsley posted about shooting two "pigs" in retaliation for the death of Eric Garner, who died after a New York police officer used a chokehold during an arrest.

An image later emerged of an Instagram post that has been attributed to Brinsley.

Part of the caption with it reads: "I'm putting wings on pigs today. They take 1 of ours...Let's take 2 of theirs."

We mourn the loss of NYPD Police Officers Wenjian Liu & Rafael Ramos. Both were shot & killed today while working in Brooklyn.

— NYPD NEWS (@NYPDnews) December 21, 2014

Police received a warning fax

Brinsley (28) shot and seriously injured his ex-girlfriend earlier on Saturday in Baltimore, Maryland, before travelling to New York.

He fired through the passenger side of the officers' vehicle with a silver semi-automatic handgun just before 3pm local time (8pm Irish time).

He then ran inside a subway station and fatally shot himself in the head as responding officers pursued him.

The shooting occurred around the same time New York police were receiving a warning fax from authorities in Baltimore.

President Barack Obama condemned the killings, saying there was no justification for them, while Attorney General Eric Holder called the deaths an "unspeakable act of barbarism".

Local resident Derrick Thompson said the shooting happened across from the Tompkins Houses public housing development.

"I was watching TV, and then I heard the helicopters," he said. "I walked out, and all of a sudden - this."

The shooting comes amid heightened tensions in New York and across the United States over police tactics following the death of Mr Garner and the shooting dead of Michael Brown in Missouri.

Several officers have been assaulted at protests in New York during demonstrations that have largely been peaceful.

Mr Bratton said they were looking at whether the suspect had attended any rallies or demonstrations.

Mayor Bill de Blasio has come under attack for not supporting the police, and on Saturday some officers turned their backs on him as he walked into the hospital where the two cops were declared dead.

The New York police union head declared there is "blood on the hands" of protesters and Mr de Blasio.

The mayor condemned the attacks:

The Reverand Al Sharpton, a civil rights leader who has supported the families of Mr Brown and Mr Garner, said Mr Garner's family has no connection to the suspect and denounced the shootings.

He said: "Any use of the names of Eric Garner and Michael Brown, in connection with any violence or killing of police, is reprehensible and against the pursuit of justice in both cases."

Mr Brown's family also condemned the shootings, saying in a statement they reject "any kind of violence directed toward members of law enforcement".

Originally posted at 6.49am