A Federal Aviation Administration spokeswoman says the Eurocopter AS350 went down around 7:15 pm (local time) in the waterway just north of Roosevelt Island and is reportedly inverted in the water.
The video posted shows a red helicopter land hard in the water and then tips over as its rotors slap at the water, sinking in the river fast.
"The Mayor was just briefed by the police and fire commissioners. At least one survivor, at least two fatalities. Divers are still in the water," tweeted Eric Phillips, New York Mayor's spokesperson.
Unconfirmed reports state that at least five people could be dead.
It was not immediately known how many people were aboard the aircraft at the time. Police harbour units are at the scene.
The FAA and the National Transportation Safety Board are investigating in the incident.
No other information has been released.
Witnesses state that the pilot managed to get and climbed aboard a raft, but others remained trapped in the sinking helicopter.