The Bond aircraft was returning from a BP-operated oil platform just before 2pm when it went down off the Aberdeenshire coast.
In a statement, Grampian Police said: "We can confirm that eight bodies have been recovered from the North Sea after a helicopter came down around 35 miles off the coast of Crimond.
"The remaining eight persons are unaccounted for."
The Maritime and Coastguard Agency said the aircraft, a Bond AS 33L Mk2, was carrying 16 people, including two crew, when it came down.
It was returning from the Miller oil and gas field, BAA said.
The MCA said in a statement: "Rescue vessels are in the process of recovering a number of fatalities following a helicopter crash earlier this afternoon. Two life-rafts were spotted in the water which were both overturned. The search is continuing for the remaining passengers and crew."
The incident comes about six weeks after a Bond Super Puma with 18 people on board ditched in the North Sea as it approached a production platform owned by BP. Everyone survived the accident.
The alarm was raised today when Aberdeen Coastguard received a mayday call at about 13.57, and a major rescue operation was launched.
Two RAF helicopters and a Nimrod were scrambled to the scene.
They were being assisted in the search by 11 vessels, including two RNLI lifeboats from Fraserburgh and Peterhead.
Emergency services were placed on stand-by and NHS Grampian put a major incident plan into action.
Grampian Police said concerned relatives are asked to contact 01224 836479.
Scotland's First Minister Alex Salmond said: "It is an awful tragedy and obviously our thoughts are with the families and their loved ones across the oil and gas community.
"The Miller field is one that's very well known to me, as are many of the people who work in it.
"Our thoughts and prayers are with the families tonight."
Mr Salmond, the MP for Banff and Buchan and MSP for Gordon in the north east, cancelled a planned trip to the Scotland v Iceland football match to go to the Grampian Police headquarters in Aberdeen tonight.
He said reports from the scene were of a "crash not a ditching" and added: "We may be looking at all 16 souls lost."
A statement will be made to the Scottish Parliament on the incident tomorrow, he said.
Cabinet secretary Kenny MacAskill went to Aberdeen shortly after the crash.
Head of fleet operations at the RNLI, Hugh Fogarty, said weather conditions in the area were believed to be good.
He added that the temperature in the North Sea was thought to be around 5C at this time of year.
"People who travel on these helicopters to and from the rigs wear proper immersion suits," he added.
"The survival time, with the proper equipment, could be a number of hours. We are certainly not giving up hope yet."
The Miller oil and gas field is 168 miles north east of Aberdeen.
Production ran from 1992 to 2007 and preparations are under way to decommission the platform.
Miller produced 345 million barrels of oil during its lifetime.
The Coastguard confirmed the helicopter was an earlier model of the one which ditched in February.
An interim report into the 18 February incident from the Air Accidents Investigation Branch (AAIB) last week said the helicopter had run into a bank of fog as it prepared to land and that the commander, Michael Tweedie, had been unable to identify the helideck of the BP platform.
The AAIB said that "black box" recorded data recovered from the sea showed that an audio voice warning telling the crew members that they were at 100ft had not come on although it was "clearly audible" in a recording of the landing at the end of the previous flight.
Investigations into the incident are continuing, but the AAIB said a preliminary examination of the recorded information and the wreckage had shown "no evidence of any pre-impact malfunction of any major mechanical components of the helicopter".