Provincial Governor Vasip Sahin confirmed the number of casualties. "The fog is strong, absolutely nothing is visible", district Mayor Hasan Akgun said in a broadcast on Turkey's NTV refuting media reports that the vehicle was a firefighting helicopter.
The Sikorsky S-76 helicopter span off-course after taking off from Istanbul's main Ataturk airport with two pilots, four foreign nationals and one Turkish citizen on-board.
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, adding that the highway had been closed down.
Authorities have launched an investigation into the crash.
Belonging to the Eczacibasi group of companies, it had been carrying a Turkish official, the firm's four Russian guests and two Turkish pilots.
The company executive was a Turkish national who headed the firm's operations in Russian Federation, he added.
The tower, which is no longer used for transmitting television signals, has a restaurant at the top it.