Thousands of foreign aid workers have arrived in Turkey and the first UN aid convoy has entered Syria after this week's twin earthquakes.
The death toll surpassed 20,000 across Turkey and neighboring Syria, with thousands still missing.
Turkey's parliament passed a three-month state of emergency in the affected areas, while President Recep Tayyip Erdogan visited the quake-hit provinces of Gaziantep, Osmaniye and Kilis.
Emergency crews used pick axes, shovels and jackhammers to dig through twisted metal and concrete — and occasionally still pulled survivors out. But in some places, they switched the focus to demolishing unsteady buildings.
While stories of miraculous rescues briefly buoyed spirits, the grim reality of the hardship facing tens of thousands who survived the disaster cast a pall. The number of deaths has surpassed the toll in a 2011 earthquake off Japan that triggered a tsunami, killing more than 18,400 people.
The death toll from Monday's 7.8-magnitude quake is expected to rise sharply as rescue efforts near 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 massive earthquake, which is 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.
"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, even as the death toll continued to rise.
Twitter was not working on Turkish mobile networks, according to AFP journalists and NetBlocks web monitoring group.
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 or banned from returning to their homes.
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.
Officials and medics said 12,391 people had died in Turkey and at least 2,992 in neighbouring Syria from Monday's quake, bringing the total to 15,383. Experts fear the number will continue to rise sharply.
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," said EU chief Ursula von der Leyen on Twitter.
"No one should be left alone when a tragedy like this hits a people," von der Leyen 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," a resident named Hassan, who did not provide his full name, said in the 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 European Union was swift to dispatch rescue teams to Turkey after the massive earthquake struck the country on Monday close to the border with Syria.
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 eastern Erzincan province.
In 1999, a 7.4-magnitude earthquake killed more than 17,000.