A helicopter has crashed into a television tower in Istanbul in dense fog, killing at least seven people.
The Sikorsky S-76 aircraft, owned by private firm Eczacibasi, crashed shortly after take off from the city's Ataturk Airport.
Taxi driver Fikret Karatekin told broadcaster CNN Turk the helicopter struck the tower before crashing on a road.
"It hit the tower and crashed by spinning," he said.
Another witness said: "I was driving. I heard an explosion... we saw pieces falling out of the sky.
"There was a lot of smoke. It exploded like a bomb. At first we thought it was a terror attack or something,"
Seven people were on board at the time - four Russians and three Turkish nationals including an executive of Eczacibasi and two pilots.
Bulent Eczacibasi. chairman of the Turkish conglomerate which is involved in pharmaceuticals and building products, did not give any details as to who had survived the crash.
But he told reporters at the scene the Russians were clients who were being taken to a factory near the town of Bozuyuk, 120 miles south of the Turkish capital, when the aircraft came down.
The company executive was a Turkish national who headed the firm's operations in Russia, he added.
Vasip Sahin, the governor of Istanbul, confirmed at least five people had been killed, adding that the cause of the crash was being investigated.
Images posted on social media showed huge plumes of black smoke billowing from the crash scene in Buyukcekmece district.
Pieces of the helicopter lay scattered across the road, which was sealed-off by police as firefighters tackled the blaze.
Authorities have not ruled out a technical fault as the cause of the crash.
The tower, which is no longer used for transmitting television signals, now has a restaurant at the top.
Thick fog in the city has led to flights and ferry services being cancelled this month.