The magnitude 7.8 quake, which hit early on Monday morning, was the worst to hit Turkey this century. It was also felt in Cyprus and Lebanon.
Rescue workers operating in bitter winter weather pulled casualties from rubble across the region.
Turkish President Tayyip Erdogan said 912 people were killed, 5383 injured, and 2818 buildings had collapsed.
Erdogan said he could not predict how much the death toll would rise as search and rescue efforts continued.
In Syria, already wrecked by more than 11 years of civil war, the health ministry said more than 326 people had been killed and 1042 injured. In the Syrian rebel-held northwest, rescuers said 147 people had died.
In Diyarbakir, Reuters journalists saw dozens of rescue workers searching through a mound of debris, all that was left of a big building, hauling off bits of wreckage as they looked for survivors. Occasionally they raised their hands and called for quiet, listening for sounds of life.
Men carried a girl wrapped in blankets from a collapsed building in the city.
Footage circulated on Twitter showed two neighbouring buildings collapsing one after the other in Syria's Aleppo, filling the street with billowing dust. Two residents of the city, which has been heavily damaged in the war, said the buildings had fallen in the hours after the quake.
Aleppo health director Ziad Hage Taha told Reuters wounded people were "arriving in waves".
Syrian state television showed footage of rescue teams searching for survivors in heavy rain and sleet.
In Afrin, a Syrian opposition-held town, rescue workers from the White Helmets response organisation searched in the pre-dawn darkness for people trapped in a collapsed building. Using only torchlight, they pulled a man in a white vest and dark trousers from the rubble, carrying him away for treatment.
In Syria, President Bashar al-Assad held an emergency cabinet meeting to review the damage and discuss the next steps, his office said.
People in Damascus and in the Lebanese cities of Beirut and Tripoli ran into the street and took to their cars to get away from their buildings in fear of collapses, witnesses said.
Footage on broadcaster CNNTurk showed the historic Gaziantep Castle was severely damaged.
Erdogan said 45 countries had offered to help the search and rescue efforts.
The United States was "profoundly concerned" about the quake and was monitoring events closely, White House national security adviser Jake Sullivan said on Twitter.
"We stand ready to provide any and all needed assistance," he said.
The US Geological Survey said quake struck at a depth of 17.9 kilometres. It reported a series of earthquakes, one of 6.7 magnitude.
The region straddles seismic fault lines.
"The combination of large magnitude and shallow depth made this earthquake extremely destructive," Mohammad Kashani, Associate Professor of Structural and Earthquake Engineering at the University of Southampton, said.
It was Turkey's most severe quake since 1999, when one of similar magnitude devastated Izmit and the heavily populated eastern Marmara Sea region near Istanbul, killing more than 17,000.
Tremors were felt in the Turkish capital of Ankara, 460km northwest of the epicentre, and in Cyprus, where police reported no damage.
"The earthquake struck in a region that we feared. There is serious widespread damage," Kerem Kinik, the chief of the Turkish Red Crescent relief agency, told Haberturk, issuing an appeal for blood donations.
Turkey is among the most earthquake-prone countries in the world. More than 17,000 people were killed in 1999 when a 7.6-magnitude quake struck Izmit, a city southeast of Istanbul. In 2011, a quake in the eastern city of Van killed more than 500.
