As the scale of the disaster became ever more apparent, the death toll looked likely to rise considerably. One U.N. official said thousands of children may have died.
Turkish President Tayyip Erdogan declared a state of emergency in 10 provinces. But residents in several damaged Turkish cities voiced anger and despair at what they said was a slow and inadequate response from the authorities to the deadliest earthquake to hit Turkey since 1999.
"There is not even a single person here. We are under the snow, without a home, without anything," said Murat Alinak, whose home in Malatya had collapsed and whose relatives are missing. "What shall I do, where can I go?"
Monday's magnitude 7.8 quake, followed hours later by a second one almost as powerful, toppled thousands of buildings including hospitals, schools and apartment blocks, injured tens of thousands, and left countless people homeless in Turkey and northern Syria.
Rescue workers struggled to reach some of the worst-hit areas, held back by destroyed roads, poor weather and a lack of resources and heavy equipment. Some areas were without fuel and electricity.
With little immediate help at hand, residents picked through rubble sometimes without even basic tools in a desperate hunt for survivors.
Aid officials voiced particular concern about the situation in Syria, already afflicted by a humanitarian crisis after nearly 12 years of civil war.
Erdogan declared 10 Turkish provinces a disaster zone and imposed a state of emergency for three months that will permit the government to bypass parliament in enacting new laws and to limit or suspend rights and freedoms.
The government will open up hotels in the tourism hub of Antalya to temporarily house people impacted by the quakes, said Erdogan, who faces a national election in three months' time.
The death toll in Turkey rose to 5,894, Vice President Fuat Oktay said. More than 34,000 were injured. In Syria, the toll was at least 1,932, according to the government and a rescue service in the insurgent-held northwest.
Turkish authorities say some 13.5 million people were affected in an area spanning roughly 450 km (280 miles) from Adana in the west to Diyarbakir in the east, and 300 km from Malatya in the north to Hatay in the south.
Syrian authorities have reported deaths as far south as Hama, some 250 km from the epicentre.
"It's now a race against time," World Health Organization Director General Tedros Adhanom Ghebreyesus said in Geneva. "Every minute, every hour that passes, the chances of finding survivors alive diminishes."
Across the region, rescuers toiled night and day as people waited in anguish by mounds of rubble clinging to the hope that friends, relatives and neighbours might be found alive
In Antakya, capital of Hatay province bordering Syria, rescue teams were thin on the ground and residents picked through debris themselves. People pleaded for helmets, hammers, iron rods and rope.
One woman, aged 54 and named Gulumser, was pulled alive from an eight-storey building 32 hours after the quake.
Another woman then shouted at the rescue workers: "My father was just behind that room she was in. Please save him."
The workers explained they could not reach the room from the front and needed an excavator to remove the wall first.
More than 12,000 Turkish search and rescue personnel are working in the affected areas, along with 9,000 troops. More than 70 countries offered rescue teams and other aid.
But the sheer scale of the disaster is daunting.
"The area is enormous. I haven't seen anything like this before," said Johannes Gust, from Germany's fire and rescue service, as he loaded equipment onto a truck at Adana airport.
Turkey's Disaster and Emergency Management Authority said 5,775 buildings had been destroyed in the quake and that 20,426 people had been injured.
Two U.S. Agency for International Development teams with 80 people each and 12 dogs are set to arrive Wednesday morning in Turkey and head to the southeastern province of Adiyaman to focus on urban search and rescue.
UNICEF spokesperson James Elder told reporters in Geneva that the earthquake "may have killed thousands of children."
Syrian refugees in northwest Syria and in Turkey were among the most vulnerable people affected, Elder said.
In the Syrian city of Hama, Abdallah al Dahan said funerals for several families were taking place on Tuesday.
"It's a terrifying scene in every sense," said Dahan, contacted by phone. "In my whole life I haven't seen anything like this, despite everything that has happened to us."
Mosques opened their doors to families whose homes were damaged.
The Syrian state news agency SANA said at least 812 people were killed in the government-held provinces of Aleppo, Latakia, Hama, Idlib and Tartous.
At least 1,120 people were killed in Syria's opposition-held northwest with the toll expected to "rise dramatically", the White Helmets rescue team said.
"There are lot of efforts by our teams, but they are unable to respond to the catastrophe and the large number of collapsed buildings," group head Raed al-Saleh said.
A U.N. humanitarian official in Syria said fuel shortages and the harsh weather were creating obstacles.
"The infrastructure is damaged, the roads that we used to use for humanitarian work are damaged," U.N. resident coordinator El-Mostafa Benlamlih told Reuters from Damascus.
In Malatya, Turkey, locals with no specialist equipment or even gloves tried to pick through the wreckage of homes crumpled by the force of the earthquake.
"My in-laws' grandchildren are there. We have been here for two days. We are devastated," said Sabiha Alinak.
"Where is the state? We are begging them. Let us do it, we can rescue them. We can do it with our means."