Five people have died after a helicopter owned by a tour group plunged into Manhattan's East River, police said on Monday.

The red chopper, which is owned by tourism group Liberty, was carrying six people including the pilot when it crashed into the river near the upscale Upper East Side neighborhood shortly after 7:00 pm (2300 GMT) on Sunday.

"Five people are deceased," a New York Police Department spokesperson told AFP, saying the dead were all passengers on the helicopter.

DEVELOPING: FDNY and NYPD responding to reports of helicopter crash in the East River off Manhattan. https://t.co/W2eF2urOcK

There are no reports of injuries at this time, authorities say. pic.twitter.com/xN3f5EBoxK

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Following the crash, "the pilot freed himself, the other five did not," Fire Commissioner Daniel Nigro told reporters during an earlier press conference.

Divers rushed to the scene and freed the passengers, two of whom were pronounced dead, while three others were taken to hospital in critical condition, Nigro said.

"We are told the five people were all tied tightly in harnesses that had to be cut and removed," he said.

Liberty organizes aerial tours of America's financial capital. Police chief James O'Neill said the aircraft had been leased by photographers.

The cause of the accident was not immediately clear, but local media indicated the pilot had sent a distress signal that mentioned an engine problem shortly before the crash.

The National Transportation Safety Board is investigating the incident.