Rescuers in Syria and Turkey dug with their bare hands through the freezing night Tuesday hunting for survivors among the rubble of thousands of buildings felled in a series of violent earthquakes.

Turkish authorities announced that the death toll has reached 5,434. In Syria, around 1,970 people were killed following the earthquake.

Turkish and Syrian disaster response teams report more than 5,600 buildings have been flattened across several cities, including many multi-story apartment blocks that were filled with sleeping residents when the first quake struck.

In the city of Kahramanmaras in southeastern Turkey, eyewitnesses struggled to comprehend the scale of the disaster.

"We thought it was the apocalypse," said Melisa Salman, a 23-year-old reporter. "That was the first time we have ever experienced anything like that."

Turkey's relief agency AFAD on Tuesday said there were now 2,921 deaths in that country alone, bringing the confirmed tally to 4,365.

There are fears that the toll will rise inexorably, with World Health Organization officials estimating up to 20,000 may have died.

In Gaziantep, a Turkish city home to countless refugees from Syria, rescuers picking through the rubble screamed, cried and clamored for safety as another building collapsed nearby without warning.

The initial earthquake was so large it was felt as far away as Greenland, and the impact is big enough to have sparked a global response.

Dozens of nations have vowed to send help, although freezing rain and sub-zero temperatures have slowed the response.

In the southeastern Turkish city of Sanliurfa, rescuers were working into the night to try and pull survivors from the wreckage of a seven-story building that had collapsed.

"There is a family I know under the rubble," said 20-year-old Syrian student Omer El Cuneyd.

"Until 11:00 am or noon, my friend was still answering the phone. But she no longer answers. She is down there."

Despite freezing temperatures outside, terrified residents spent the night on the streets, huddling around fires for warmth.

Mustafa Koyuncu packed his wife and their five children into their car, too scared to move.

"We can't go home," the 55-year-old told AFP. "Everyone is afraid."

Some of the heaviest devastations occurred near the quake's epicenter between Kahramanmaras and Gaziantep, where entire city blocks lay in ruins under gathering snow.