At least two people have died after a tour helicopter crashed into the East River off Manhattan in New York, AP reports.

Eric Phillips, a spokesman for Mayor Bill de Blasio, said in a post on Twitter there was "at least one survivor, at least two fatalities. Divers are still in the water."

The New York Post reports that five passengers were trapped and died while the pilot "scrambled to safety".

Reports have put the number of people on the Eurocopter AS350 chopper at between one and six. The FAA said the helicopter was upside down.

The chopper went down near Gracie Mansion about 7.15pm, a high-ranking source told the Post.

The Post said the pilot managed to get and climb onto a raft. Police deployed two divers, and the person was taken to Bellevue Hospital, sources told the Post. The pilot had called in a emergency to the La Guardia Airport air-traffic control tower, sources told the Post.

"There was about a minute where we didn't see anyone come out, then after a minute we saw a man come out holding onto a raft screaming, 'Help! help! Ah!' and then inaudible screams," witness Brianna Jesme told the Post. "It was horrifying."

CBS posted video which it said showed a survivor emerging from the crash.

The mayor's office said divers with the NYPD and FDNY were searching in the water. The crash happened near East 90 St and north of Roosevelt Island.

Video posted to Twitter shows a red helicopter splash down in the river.

Within minutes, the helicopter disappeared beneath the waves, witness John Magers, 24, told the Post.

"It looked like it was completely submerged," he said. "It couldn't have been any longer than 5 minutes."

The person who escaped was scooped up by a tug boat, sources said.

Magers said he saw police drop divers into the river.

A witness told NY1, "We heard it hit the water, and then automatically, it drowned."

Her friend added, "It landed to the side, and then it flipped over, and then for about a minute and a half, no one came out of the water, so all of us immediately called 911. And then we saw one guy climb onto the raft."

Witness Mary Lee, 66, told the Post that she saw the chopper go down from her apartment window and said she "immediately thought someone was in there and not going to live — it's cold water, it was sinking really fast.

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

The Post reported that there are only two Eurocopter AS350's registered with the FAA in the area — both to the New Jersey parent company of tour operator Fly Liberty Charters, FAA records show. Fly Liberty could not immediately be reached for comment.