Turkish President Recep Tayyip Erdogan on Wednesday conceded "shortcomings" after criticism of his government's response to the massive earthquake that has killed over 15,000 people in Turkey and Syria.

The sprawling scale of the disaster that flattened thousands of buildings, trapping an unknown number of people, has swamped relief operations already hampered by freezing weather.

Survivors have been left to scramble for food and shelter -- and in some cases watch helplessly as their relatives called for rescue, and eventually went silent under the debris, AFP reports.

"My nephew, my sister-in-law and my sister-in-law's sister are in the ruins. They are trapped under the ruins and there is no sign of life," said Semire Coban, a kindergarten teacher, in Turkey's Hatay.

"We can't reach them. We are trying to talk to them, but they are not responding... We are waiting for help. It has been 48 hours now," she said.

Still, searchers kept pulling survivors from the debris three days after the 7.8 magnitude quake that is already one of the deadliest this century, even as the death toll continues to rise.

As criticism mounted online, Erdogan visited one of the hardest-hit spots, quake epicentre Kahramanmaras, and acknowledged problems in the response.

"Of course, there are shortcomings. The conditions are clear to see. It's not possible to be ready for a disaster like this," he said.

Twitter was also not working on Turkish mobile networks, according to AFP journalists and NetBlocks web monitoring group.