Pilot's mayday call suggests engine failure as New York City East River helicopter crash leaves five dead

An emergency call from the pilot of a helicopter which crashed into New York City's East River, killing all five passengers, suggests that the craft was experiencing engine failure just before it went down and flipped over.

The pilot of the helicopter, which had been chartered for a photo shoot, called mayday several times before the crash.

In the emergency call, the pilot can be heard radioing: "Mayday! Mayday! Mayday!... East River. Engine Failure."

The pilot freed himself and was rescued by a tugboat after the helicopter went down around 7pm local time (11pm GMT) on Sunday night.

However, the passengers were still tightly strapped to their seats when police and fire department divers found them, and had to be cut free.

Fire Commissioner Daniel Nigro confirmed that the passengers were still strapped in and upside down, with the divers having to remove them from tight harnesses.

Two of the passengers died at the scene, while the three others were in critical conditions and later died in hospital.

"It took awhile for the divers to get these people out. They worked very quickly as fast as they could," Mr Nigro said.

"It was a great tragedy that we had here."

The tragic crash was caught on camera as the helicopter hit water in a stretch of the river near the New York mayor's Gracie Mansion residence.

The red helicopter can be seen in the footage flipping on its side as its rotors strike the water.

In an air traffic control radio conversation, an operator confirms that the helicopter is "upside down".

Three passengers were taken to hospital in critical condition but later died. Credit: AP

Witnesses said just before the crash, the helicopter had been flying noisily and then it suddenly dropped into the water and was quickly submerged.

They continued that the pilot then appeared on the surface, holding onto a flotation device as a tugboat and then police boats approached.

"It's cold water. It was sinking really fast," Mary Lee, 66, told the New York Post.

"By the time we got out here, we couldn't see it. It was underwater."

The helicopter was being chartered for a photo shoot. Credit: AP

The names of the pilot and the passengers have not yet been released by officials.

The helicopter has been recovered and towed to a pier.

A Federal Aviation Administration spokeswoman said the Eurocopter AS350 was owned by Liberty Helicopters.

The firm offers both private charters and sightseeing tours.