Five people have been killed after a helicopter crashed into a highway in the outskirts of Istanbul after reportedly hitting a TV tower in dense fog.

The Sikorsky S-76, belonging to the Eczacibasi group of companies,was carrying four Russian guests, a Turkish official, and two pilots, reported the Associated Press.

The emergency centre for Russians abroad said the four Russians on board were amongst those killed. It is not known who the fifth victim was.

Pictures and footage shows thick smoke billowing from the flaming wreckage spread across the motorway in Buyukcekmece, on the outskirts of Istanbul.

Eyewitness Fikret Karatekin, a taxi driver, told CNN-Turk television by telephone that the helicopter slammed into the tower before crashing on the highway.

"It hit the tower and crashed by spinning," he said.

The tower, which is no longer used for transmitting signals, has a restaurant at the top. The structure is 236 meters (774ft) high with a viewing platform at 160 meters.

The helicopter had reportedly taken off from Istanbul's Ataturk airport and those on board were being transported to visit the company's factory near the town of Bozuyuk, some 200km (120 miles) south of Istanbul.