Two US police officers shot dead while on duty

Two US police officers have died after being shot execution style while sitting in a patrol car in Brooklyn, New York.

Gunman Ismail Brinsley ambushed the officers in broad daylight outside of the Tompkins Houses in Bedford-Stuyvesant.

The officers were working overtime as part of an anti-terrorism drill.

The 28-year-old ran to a nearby underground station, but when police closed in he turned the gun on himself on the crowded platform.

Police soon discovered the shooting was a revenge attack for police involved in killings of unarmed black men.

"With no warning, with no provocation they were quite simply assassinated," NYPD Commissioner William Bratton said.

Brinsley had posted a photo of a handgun on Instagram just three hours earlier, with the caption "I'm putting wings on pigs today".

Two other photos of the gun included the captions "They Take 1 Of Ours … Let's Take 2 of Theirs" and "This May Be My Final Post".

Brinsley was already on the run after murdering his girlfriend earlier that day.

"When a police officer is murdered it tears at the foundation of our democracy and society," New York City Mayor Bill De Blasio said.

"Our prayers are with our fellow #NYPD brothers who were executed in the line of duty today in #Brooklyn," the New York Police Department's 66th Precinct posted on its official Twitter page.

NYPD Captains Endowment Association president Roy Richter called the assault the "worst nightmare."

"Incredible sadness as we mourn for our brothers and their families," he added on Twitter.