The US Geological Survey said the 7.8 magnitude tremor struck at 4.17am (local time) at a depth of 17.9km near the city of Gaziantep.
It said on Twitter extensive damage was probable.
The earthquake was felt in the capital Ankara and other Turkish cities, and also across the region.
Reports are coming in that several buildings have collapsed, and a number of people may be trapped.
A BBC Turkish correspondent in Diyarbakir reported that a shopping mall in the city had collapsed.
Rushdi Abualouf, a BBC producer in the Gaza Strip, said there was about 45 seconds of shaking in the house he was staying in.
Turkish seismologists estimated the strength of the quake to be 7.4 magnitude.
They said that a second tremor hit the region just minutes later.
Turkey's Disaster and Emergency Management Authority (AFAD) pegged the quake at 7.4 near the southern city of Kahramanmaras.
There was damage to buildings and people were gathered out on snowy streets, according to images on state broadcaster TRT.
The earthquake lasted about a minute and shattered windows, according to a Reuters witness.
The area is regularly hit by strong earthquakes.
Gokhan Cali/Anadolu Agency/Getty Images Caption: A view of the destroyed building after 7.4 magnitude earthquake jolts Turkiye's Kahramanmaras province, on February 6, 2023 in Kahramanmaras, Turkiye.