The magnitude 7.8 quake, which hit in the early darkness of a winter 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.
"We were shaken like a cradle. There were nine of us at home. Two sons of mine are still in the rubble, I'm waiting for them," said a woman with a broken arm and injuries to her face, speaking in an ambulance near the wreckage of a seven-storey block where she had lived in Diyarbakir in southeast Turkey.
Turkish President Tayyip Erdogan said 912 people were killed, 5,383 injured, and 2,818 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 1,042 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.
"We woke up to a big noise and severe shaking. There were two aftershocks right after that," said Meryem, 29, from the southeastern Turkish city of Kahramanmaras, near the epicentre.
"I was so scared, thought it will never stop. I took some things for my one-year old son and left the building."
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.
'LIKE THE APOCALYPSE'
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 organization 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 the border town of Azaz - another rebel-held area - a rescue worker carried a toddler from a damaged building.
"It was like the apocalypse," said Abdul Salam al Mahmoud, a Syrian contacted by Reuters from the town of Atareb.
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.
In the Turkish city of Malatya, a rescue worker crawled into a collapsed building, trying to identify a survivor trapped under the wreckage, in footage released by Turkey’s Disaster and Emergency Management Authority (AFAD).
"What colour are you wearing? Are you wearing pink? Please take care of yourself for the moment, I cannot see anything else," the rescue worker could be heard saying.
WORST QUAKE SINCE 1999
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 U.S. Geological Survey said quake struck at a depth of 17.9 km. 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, 460 km (286 miles) northwest of the epicentre, and in Cyprus, where police reported no damage.