Frantic rescue efforts are continuing as hundreds of people are trapped under rubble following a powerful 7.8 magnitude earthquake early Monday morning that rocked south-eastern Turkey and northern Syria killing more than 2,300.
The death toll is expected to rise as rescue workers search the wreckage in cities and towns across the region.
At least 20 aftershocks followed, some hours later during daylight, the strongest with a magnitude of 6.6, Turkish authorities said.
Rescue workers and residents in multiple cities have been searching for survivors, working through tangles of metal and giant piles of concrete.
Turkey-Syria earthquake - latest updates
A hospital in Turkey also collapsed and patients, including newborn babies, were evacuated from a handful of facilities in Syria.
Turkish President Recep Tayyip Erdogan said: "Because the debris removal efforts are continuing in many buildings in the earthquake zone, we do not know how high the number of dead and injured will rise.
"Hopefully, we will leave these disastrous days behind us in unity and solidarity as a country and a nation."
The quake, felt as far away as Cairo, was centred on Turkey's south-eastern province of Kahramanmaras.
It struck a region that has been shaped on both sides of the border by more than a decade of civil war in Syria.
Latest figures from Turkey's disaster agency show 1,498 fatalities have been recorded in 10 provinces, with some 7,600 injured.
Timing of quake is behind rapidly rising death toll - and why it will be tough to get aid to Syria Alistair Bunkall@AliBunkallSKY
The images coming out of southern Turkey and northwest Syria are grim.
The earthquake struck before dawn, when most people were in bed, asleep.
That factor will likely add to the rapidly increasing death toll, as will severe aftershocks.
The coming hours will be crucial as rescue workers race against time to locate survivors. Already Turkey has declared a state of emergency and help is being pledged from around the world.
The situation in northern Syria is especially concerning. The region has already suffered 12 years of civil war which has left many buildings damaged and weakened, and there are hundreds of thousands of refugees displaced by fighting.
Getting aid into this contested part of Syria will be a challenge in itself.
There is a major aid hub nearby in Dubai, where warehouses are full of medical and humanitarian supplies ready to fly if access to Turkey and Syria can be negotiated.
Turkey, which sits on a fault line, has a history of earthquakes and therefore will have some expertise already on the ground, but this is already looking like a major disaster that will need all the international help available.
In Syria's government-controlled areas, a total of 371 people have died.
The Syrian Civil Defence, known as the White Helmets, has confirmed 380 fatalities in opposition-held areas, which are packed with about four million people displaced from other parts of the country by the fighting.
Hundreds of families remain trapped in the rubble, according to the White Helmets.
Strained health facilities and hospitals were quickly filled with wounded, rescue workers said. Others had to be emptied, including a maternity hospital, according to the SAMS medical organisation.
"We fear that the deaths are in the hundreds," Dr Muheeb Qaddour said by phone from the town of Atmeh, in northern Syria.
Buildings were reported collapsed in a wide area extending from Syria's cities of Aleppo and Hama to Turkey's Diyarbakir, more than 200 miles to the north-east.
Nearly 900 buildings were destroyed in Turkey's Gaziantep and Kahramanmaras provinces, said vice president Fuat Oktay.
A hospital collapsed in the Mediterranean coastal city of Iskenderun, but casualties were not immediately known, he said.
"Unfortunately, at the same time, we are also struggling with extremely severe weather conditions," Mr Oktay told reporters.
Nearly 2,800 search and rescue teams have been deployed in the disaster-stricken areas, he added.
The US Geological Survey measured Monday's quake at 7.8. Hours later, a 7.5 magnitude one struck more than 60 miles away.
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Analysis: This is the worst kind of quake
Why is the earthquake death toll so high in Turkey and Syria?
An official from Turkey's disaster management agency said it was a new earthquake, not an aftershock, though its effects were not immediately clear.
The quake heavily damaged Gaziantep's most famed landmark, its historic castle perched on a hill in the centre of the city.
Parts of the fortresses' walls and watch towers were levelled and other parts heavily damaged.
In Diyarbakir, hundreds of rescue workers and civilians formed lines across a mountain of wreckage, passing down broken concrete pieces, household belongings and other debris as they searched for trapped survivors.
In north-west Syria, the quake added new woes to the opposition-held enclave centred on the province of Idlib.
The opposition's Syrian Civil Defence described the situation there as "disastrous", adding that entire buildings have collapsed and people are trapped under the rubble.
In the small Syrian rebel-held town of Azmarin in the mountains by the Turkish border, the bodies of several dead children, wrapped in blankets, were brought to a hospital.
Mr Erdogan said early on Monday that 45 countries had offered help with search and rescue efforts.
Foreign Secretary James Cleverly said the UK government would be "sending immediate support", with a team of 76 search-and-rescue specialists, equipment and four search dogs being sent to Turkey.
Prime Minister Rishi Sunak tweeted: "My thoughts are with the people of Turkiye and Syria this morning, particularly with those first responders working so valiantly to save those trapped by the earthquake.
"The UK stands ready to help in whatever way we can."
Turkey sits on top of major fault lines and is frequently shaken by earthquakes.
At least 18,000 were killed in powerful earthquakes that hit northwest Turkey in 1999.
Stephen Hicks, a seismologist at University College London, told Sky News that Turkey and Syria have experienced "the worst kind of earthquake".
Turkey's deadly history of earthquakes
Turkey and the surrounding area have suffered several devastating earthquakes in recent years with thousands of lives lost.
More than 1,300 people have died in the quake that hit in the early hours of this morning. Dozens of aftershocks have also been felt.
At 7.8-magnitude, it is the strongest earthquake in Turkey since the Erzincan quake in December 1939, which killed around 32,000 people.
The area sits on the Anatolian Plate, which borders two major faults - the North Anatolian fault lies from west to east in Turkey, while the East Anatolian fault is situated in the country’s south-eastern region.
Some of the deadliest earthquakes in the region have taken place in the past few decades.
30 October 2020 - A 7.0-magnitude earthquake hit the Aegean Sea with its epicentre near the Greek island of Samos. Turkey’s third-largest city Izmir was heavily affected, with 119 people killed in total and more than 1,050 injured.
24 January 2020 - More than 40 people were killed and more than 1,600 injured in a 6.7-magnitude earthquake in the eastern province of Elazig. Tremors were also felt in Syria, Lebanon and Iran.
23 October 2011 - More than 600 people were killed when a 7.2-magnitude quake struck the eastern cities of Van and Ecris. A second earthquake struck just around two weeks later which left around 40 people dead and hundreds more injured.
1 May 2003 - More than 160 people were killed, including 83 children in a collapsed school dormitory, in a 6.4-magnitude quake. About 1,000 people were injured in the disaster in the eastern city of Bingol.
12 November 1999 - In the north-western town of Ducze, nearly 1,000 people were killed by a 7.2-magnitude earthquake.
17 August 1999 - More than 17,000 people were killed in a quake that struck the western city of Izmit, around 55 miles southeast of Istanbul. Around half a million people were left homeless after the disaster.
"It's a very shallow earthquake beneath a highly populated area, a very strong earthquake, and in a region where we can see the buildings just can't withstand this level of shaking."
Mr Hicks said there is a "small chance" there could be "stronger aftershocks" or even another earthquake "larger than the main shock".