WASHINGTON (AFP) - New Orleans police launched a major hunt for a man who shot dead a veteran officer while being transported in a police car on Saturday and then fled, authorities said.

Officer Daryle Holloway, 45, was driving a suspect to jail who had been arrested the previous night on an aggravated assault charge when the incident happened.

Holloway was rushed to a hospital but died from his injuries, police chief Michael Harrison told reporters, saying that the search was on for Travis Boys, 33.

A somber Harrison said Holloway was a highly respected officer of 22 years service and described it as "an extremely sad day."

"He shot and killed our officer. And for that he will be held accountable," said Harrison.

Harrison described how Boys somehow managed to get his handcuffed hands from behind to in front of him - perhaps by being double-jointed - and through a gap in the cage that separates the back and front seats.

"He was able to produce a firearm and actually shot officer Holloway," who then crashed into a pole, said Harrison.

It was not clear where Boys got the firearm from but it was not the policeman's, he added.

Holloway attempted to stop Boys from fleeing but was unable to.

"We have a large manhunt to look for this subject," said Harrison, calling Holloway "very lively, full of personality."

"It is extremely sad. I had an opportunity to meet with every member of his family at the hospital. Obviously, they are taking it extremely hard."

The suspect and Holloway were the only two in the vehicle at the time.