A Russian helicopter with eight people on board crashed at sea in Norway's Svalbard archipelago in the Arctic on Thursday, Norwegian rescue services said, with fears growing over the fate of those on board.

None of the eight aboard had been found six hours after the accident was reported. According to Russian media, they are all Russian nationals.

Chances of survival were fading rapidly, with air and water temperatures in the region currently around zero degrees Celsius (32 Fahrenheit).

Rescuers were in a "race against the clock," rescue chief Tore Hongset told Norwegian television TV2.

"Time is passing and the risk of death by hypothermia is increasing exponentially," he said.

"Until we really know what happened, the search continues as if there is a chance of finding these people alive," he added.

The aircraft went down in the afternoon two or three kilometres from Barentsburg, a Russian mining community in the archipelago, the rescue services said.

No contact had been made with the chopper before it went down or immediately after it was reported down.

Two Norwegian helicopters and several rescue vessels were dispatched to the scene.

So far, only two oil patches have been detected in the area, but it is not known if they come from the Russian helicopter, another rescue service official told AFP.

"We're continuing the search until we've cleared this up," he said.

According to Hongset the early indications, including the oil patches, suggest that the aircraft crashed into the sea.

A sonar detection also found an object lying on the seabed, but this had not been identified, he added.

The water is 200 metres deep in the area and a robot submarine would be needed to investigate the sonar finding.

- Five crew, three scientists -

Visibility in the area was reduced because of snow and darkness, which falls early this time of year at this latitude.

The aircraft was a Russian Mil Mi-8 based permanently near Barentsburg.

It was reported missing by airport authorities around 3:35 pm (1335 GMT) as it was returning from Pyramiden, another former mining community that is now a tourist site.

It was confirmed to have come down about 10 minutes later, rescue services said.

Russia's consul general in Svalbard, Viacheslav Nikolayev, told the Interfax news agency there were five crew members and three scientists on board.

"It's a civilian helicopter that belongs to the Arktikugol company and it flies the miners between (Svalbard's main town of) Longyearbyen and Barentsburg. It's a regular flight," he said.

"We know the helicopter failed to return. But whether it crashed or not, I can't say," he said.

Norway was afforded sovereignty of Svalbard, located around 1,000 kilometres (620 miles) from the North Pole, under the 1920 Treaty of Paris.

Nationals of all signatory states enjoy "equal liberty of access and entry" to Svalbard and its waters.

As a result, Russia operates a coal mine in Barentsburg, a community home to several hundred Russian and Ukrainian miners.

In 2008, another Mil Mi-8 crashed near Barentsburg, killing three of its nine occupants. Yet another crash near Pyramiden of the same type of helicopter left two dead in 1991.