A Finnish helicopter carrying 14 people crashed into the Baltic Sea off Estonia on Wednesday and Estonian officials said all on board were believed dead.

The helicopter, on a scheduled commercial flight to Helsinki from the Estonian capital Tallinn, crashed near the Baltic island of Naissaar three minutes after take-off, officials said.

It was not clear why the helicopter suddenly dived into the choppy sea, rescue officials and the helicopter company said.

Estonian Kanal 2 television quoted eyewitness Mati Ojane, a harbour pilot in the port of Rohuneeme five km (three miles) from the crash site, as saying he had heard "two loud bangs" before seeing the aircraft go down.

Rescue helicopters, boats and divers from Estonia and Finland found debris from the helicopter but efforts to recover those on board -- six Finnish passengers, two Finnish crew, four Estonians and two Americans -- were postponed until Thursday because of strong winds and waves several metres high.

Estonian Prime Minister Andrus Ansip told a news conference in Tallinn there was not yet full information about the fate of the 14 on board, "but there is no hope".

Rein Porro, deputy director of Estonia's civil aviation authority, called it the worst civil aviation accident in Estonia's history.

"They are believed dead. Typically people cannot survive a crash like this. The helicopter dived very quickly," he said by phone from Tallinn.

Estonian officials said the helicopter had sunk, with the 14 still inside, in water about 60 metres (200 feet) deep.

Finnish police sent forensic experts to Estonia to help identify any of those on board the aircraft, a twin-engine Sikorsky S-76.

Some ferries between Tallinn and Helsinki had earlier been cancelled due to storm warnings after heavy weather and gales swept up the Baltic on Tuesday evening.

But Copterline, the helicopter's owner, ruled out poor weather as a factor in the crash.

"The helicopter was technically in very good shape. We do not know of any technical or weather-related issues that could have caused problems," Chief Executive Kari Ljungberg told a news conference.

Copterline flies the 18-minute hop between Tallinn and Helsinki 28 times a day.