The death toll from a huge earthquake that hit Turkey and Syria climbed to more than 17,100 on Thursday, as hopes faded of finding survivors stuck under rubble in freezing weather.

Officials and medics said 14,014 people had died in Turkey and 3,162 in Syria from Monday's 7.8-magnitude tremor, bringing the confirmed total to 17,176.

Freezing temperatures deepened the misery Thursday for survivors.

The death toll from Monday's 7.8-magnitude quake is expected to rise sharply as rescue efforts pass the 72-hour mark that disaster experts consider the most likely period to save lives.

Turkish President Recep Tayyip Erdogan on Wednesday conceded "shortcomings" after criticism of his government's response to the earthquake, one of the deadliest this century.

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.

"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 province.

"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, rescuers kept pulling survivors from the debris as the death toll continued to rise.

As criticism mounted online, Erdogan visited one of the hardest-hit spots, the quake's 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 access returned on Thursday morning after the social network did not work on Turkish mobile networks for several hours Wednesday, according to AFP journalists and the NetBlocks web monitoring group.

Turkish officials had held talks with Twitter leaders after which deputy infrastructure minister Omer Fatih Sayan tweeted Thursday that Turkey expected the social network to cooperate more in the "fight against disinformation".

- Children saved -

Temperatures plunged to minus-five degrees Celsius (23 degrees Fahrenheit) in Gaziantep early Thursday, but the cold did not stop thousands of families from spending the night in cars and makeshift tents, too scared to stay in their homes or prohibited from returning to them.

Parents walked the streets of the southeastern Turkish city -- close to the epicentre of the earthquake -- carrying their children in blankets because it was warmer than sitting in a tent.

"When we sit down, it is painful, and I fear for anyone who is trapped under the rubble in this," said Melek Halici, who wrapped her two-year-old daughter in a blanket as they watched rescuers working late into Wednesday night.

In Brussels, the EU is planning a donor conference in March to mobilise international aid for Syria and Turkey.

"We are now racing against the clock to save lives together," EU chief Ursula von der Leyen said on Twitter.

"No one should be left alone when a tragedy like this hits a people," she said.

- 'People dying every second' -

Due to the scale of the damage and the lack of help coming to certain areas, survivors said they felt alone in responding to the disaster.

"Even the buildings that haven't collapsed were severely damaged. There are now more people under the rubble than those above it," Hassan, who did not provide his full name, said in his rebel-held Syrian town of Jindayris.

"There are around 400-500 people trapped under each collapsed building, with only 10 people trying to pull them out. And there is no machinery," he added.

The White Helmets, leading efforts to rescue people buried under rubble in rebel-held areas of Syria, have appealed for international help in their "race against time".

They have been toiling since the quake to pull survivors out from under the debris of dozens of flattened buildings in northwestern areas of war-torn Syria that remain outside the government's control.

A leading UN official called for the facilitation of aid access to rebel-held areas in the northwest, warning that relief stocks will soon be depleted.

"Put politics aside and let us do our humanitarian work," the UN's resident Syria coordinator El-Mostafa Benlamlih told AFP in an interview.

- Syria appeals for EU help -

The issue of aid to Syria is a delicate one, and the sanctioned government in Damascus made an official plea to the EU for help, the bloc's commissioner for crisis management Janez Lenarcic said.

A decade of civil war and Syrian-Russian aerial bombardment had already destroyed hospitals, collapsed the economy and prompted electricity, fuel and water shortages.

The European Commission is "encouraging" EU member countries to respond to Syria's request for medical supplies and food, while monitoring to ensure that any aid "is not diverted" by President Bashar al-Assad's government, Lenarcic noted.

Dozens of nations, including the United States, China and the Gulf States have pledged to help, and search teams as well as relief supplies have already arrived.

The EU was swift to dispatch rescue teams to Turkey, but it initially offered only minimal assistance to Syria because of EU sanctions imposed since 2011 on Assad's government over its brutal crackdown on protesters that spiralled into a civil war.

The Turkey-Syria border is one of the world's most active earthquake zones.

Monday's quake was the largest Turkey has seen since 1939, when 33,000 people died in the eastern Erzincan province.

In 1999, a 7.4-magnitude earthquake killed more than 17,000.