Norwegian authorities said Friday that 13 people were killed when a helicopter transporting rig workers crashed off Norway's coast.

Eleven dead bodies were recovered by rescue workers and two missing people are presumed to have also died in the accident.

The state-owned Statoil said the Airbus Super Puma helicopter was en route from an offshore oil field in the North Sea to Bergen, about 120 kilometers (74 miles) away from the rig.

An official at the Norwegian Civil Aviation Authority told Norwegian daily VG that the crashed machine's maintenance had been delayed twice. "I can confirm that this specific helicopter had seen its maintenance delayed. It is correct that there was an application for a so-cakked travel-time extension," Hege Aalstad said.

"The first one was for a delay of 100 flying hours...and the other was also for 100 flying hours," he added.

Statoil added that all helicopters of the same model had been grounded. The Norwegian Aviation Authority has likewise banned all Super Puma EC225LP helicopters from flying "until further notice."

'Horrifying'

Norwegian Prime Minister Erna Solberg wrote on Twitter about the "horrifying reports," adding that she was being kept informed about rescue efforts.

Norwegian police spokesman Morten Kronen said Friday that the helicopter "has crashed, it is totally smashed" on the island of Turoey near Norway's second city.

Several witnesses described seeing a powerful explosion and people were seen in the sea, television station TV2 reported.

Eyewitness Rebecca Andersen told the newspaper Verdens Gang that the helicopter's rotor detached and that the "blades came rushing toward us."

"Then we heard a violent explosion," Andersen added.

The North Sea's dangers

In December, an energy-industry worker was killed when extreme weather hit a Statoil rig in the North Sea, reported the Wall Street Journal.

However, the Norwegian Petroleum Safety Authority said it was the first fatality on the continental shelf since 2009.

That same month, oil company BP Norway removed its personnel from oil platforms in the North Sea after an unmanned barge broke loose, threatening several rigs in the area.

rs/ls/jar/kms (Reuters, AP, AFP, dpa)