The coastguard is responding to reports that a helicopter has ditched into the sea near Shetland.
Lifeboats from Aith and Lerwick have been sent to the scene of the reported crash, two miles west of Sumburgh airport on Shetland.
A coastguard spokesman said they received a distress call at around 7pm.
The BBC reported that the helicopter was carrying 18 people and that a ferry travelling between Shetland and Aberdeen has been diverted to the scene.
The aircraft - reportedly operated by the company CHC - was said to have been taking workers to and from oil and gas platforms in the North Sea.
Police Scotland said they are aware of the incident and are working with the coastguard.
It is not known if there are any casualties or which type of helicopter is involved.
Last year, two helicopters ditched in the North Sea only six months apart. All passengers and crew were rescued in both incidents which were found to be caused by gearbox problems.
In October, 17 passengers and two crew were rescued from life rafts by a passing vessel after the helicopter, which was carrying an oil crew from Aberdeen to a rig 86 miles north west of Shetland, was forced to ditch.
Previously, in May 2012 all 14 passengers and crew members on a Super Puma helicopter were rescued after it ditched about 30 miles off the coast of Aberdeen. The helicopter was on a scheduled flight from Aberdeen Airport to a platform in the North Sea at the time.
Super Puma EC 225s were grounded in the wake of the two incidents but were given approval to fly again and services resumed earlier this month.
In April 2009, 16 people died when a Super Puma plunged into the sea. Its gearbox failed while carrying the men to Aberdeen.
The Bond-operated aircraft was returning from the BP Miller platform when it went down off the Aberdeenshire coast on April 1, 2009.
Investigators from the Air Accidents Investigation Branch (AAIB) were travelling to the scene this evening.
A spokesman for the Department for Transport said: "The AAIB is aware of the incident and has deployed a team".
The BBC reported that a coastguard rescue helicopter that was sent to the scene returned to Lerwick with nine passengers.
Eight people walked off the aircraft and one person was carried off on a stretcher, the broadcaster reported.
A spokeswoman for Northlink ferries said at least one ferry and possibly two had been diverted to help with the rescue.
"The Scottish Coastguard are in the lead of the operation, but I can confirm that at least one of our ferries is providing assistance," she said. "There will definitely have been passengers aboard that ferry."
The ferry is believed to have been travelling between Shetland and Orkney and is thought to have been carrying up to 100 passengers.