OSLO, Jan. 27 (Xinhua) -- A helicopter crashed into icy seawater in southeast Norway on Wednesday and the four people on board were declared missing, the Norwegian news agency NTB reported.

The helicopter, which was on a commercial taxi mission, fell into ice-covered sea off the Norwegian town Horten on the western side of the Oslofjorden Fjord, which lies about 57 kilometers south of the Norwegin capital city Oslo.

What is believed to be parts from the ill-fated Robinson R44 helicopter was found on ice and divers, rescue boats, a mini- submarine and rescue helicopters were sent to the scene to conduct a rescue operation.

It was too foggy in the area surrounding the crash site for the rescue helicopters to perform their tasks, Anders Bang- Andersen, head of communications at the Rescue Coordination Center, told Xinhua.

The helicopter, which was owned by the Midtnorsk Helikopterservice AS company, was flying in formation with another helicopter before its crash. Witnesses said that the helicopter was seen spinning 150 meters up from the sea surface and its propellers hit ice before it sunk into 10 meter deep sea water.

A spokesperson for the company said that the helicopter was out on a commercial taxi mission with a pilot and three customers on board.

The cause of the accident was not immediately known.