Hundreds of people are still believed to be trapped under rubble, with the death toll expected to rise further as rescue workers search mounds of wreckage in cities and towns across the area.
On both sides of the border, residents were jolted out of sleep by the pre-dawn quake rushed outside on a cold winter’s night.
Nearly 1,500 people were killed in ten Turkish provinces, with some 8,500 injured, according to the president of the country’s disaster management agency.
The death toll in government-held areas of Syria climbed past 430 people, with some 1,280 injured, according to the health ministry.
In the country’s rebel-held north-west, groups that operate there said the death toll was at least 380, with many hundreds more injured.
UK foreign secretary James Cleverly said there were no reports of British fatalities, but acknowledged the relief effort was still at an early stage. “We don’t know the full extent of the injuries or fatalities and sadly they are likely to grow over the coming days,” he said.
A British team of search and rescue specialists was expected to arrive in Turkey by 11pm UK time, according to the Government.
Buildings were reduced to piles of pancaked floors, and major aftershocks or new quakes, including one nearly as strong as the first, continued to rattle the region.
Rescue workers and residents in multiple cities searched for survivors, working through tangles of metal and concrete. A hospital in Turkey also collapsed, and patients, including newborn babies, were evacuated from 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, which was centred on Turkey’s south-eastern province of Kahramanmaras, was felt as far away as Cairo. It sent residents of Damascus rushing into the street, and jolted awake people in their beds in Beirut.
In the Turkish city of Adana, one resident said three buildings near his home were toppled. The tremor struck a region that has been shaped on both sides of the border by more than a decade of civil war in Syria.
On the Syrian side, the region affected is divided between government-held territory and the country’s last opposition-held enclave, which is surrounded by Russian-backed government forces. Turkey, meanwhile, is home to millions of refugees from that conflict.
The opposition-held regions in Syria are packed with some four million people displaced from other parts of the country by the fighting. Many of them live in buildings that are already wrecked from past bombardments. Hundreds of families remained trapped in rubble, according to the opposition emergency organisation, the White Helmets.
Strained health facilities and hospitals were quickly filled with injured, rescue workers said. Others had to be emptied, including a maternity hospital, according to the SAMS medical organisation.
The region sits on top of major fault lines and is frequently shaken by earthquakes. Some 18,000 people were killed in a similarly powerful earthquakes that hit north-west Turkey in 1999.
The US Geological Survey measured Monday’s quake at 7.8. Hours later, a 7.5 magnitude one struck more than 60 miles away. An official from Turkey’s disaster management agency said it was a new earthquake, not an aftershock, though its effects were not immediately clear. Hundreds of aftershocks were expected after the two tremors, the official said.
Thousands of 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. A hospital collapsed in the Mediterranean coastal city of Iskenderun, but casualties were not immediately known, Turkish vice president Fuat Oktay said.
Televisions stations in Turkey aired screens split into four or five, showing live coverage from rescue efforts in the worst-hit provinces. In the city of Kahramanmaras, rescuers pulled two children alive from the rubble, and one could be seen lying on a stretcher on the snowy ground.
Offers of help – from search-and-rescue teams to medical supplies and money – poured in from dozens of countries, as well as the European Union and Nato.
In Turkey, people trying to leave the quake-stricken regions caused traffic jams, hampering efforts of emergency teams trying to reach the affected areas. Authorities urged residents not to take to the roads. Mosques around the region were opened to provide shelter for people unable to return to damaged homes amid temperatures that hovered around freezing.
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 while diggers sifted through the rubble below.
The quake damaged a historic castle perched on top of a hill in the centre of the provincial capital of Gaziantep, about 20 miles from the epicentre. Parts of the fortresses’ walls and watch towers were levelled and other parts heavily damaged, images from the city showed.
Meanwhile, the directorate-general of Antiquities and Museums in Syria said the earthquake has caused some damage to the Crusader-built Marqab, or Watchtower Castle, on a hill overlooking the Mediterranean. Parts of a tower and some walls collapsed.
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 seen being brought to a hospital.
In north-west Syria, the quake added new woes to the opposition-held enclave centred on the province of Idlib, which has been under siege for years, with frequent Russian and government air strikes. The territory depends on a flow of aid from nearby Turkey for everything from food to medical supplies.
The opposition’s Syrian Civil Defence described the situation there as “disastrous”. The USGS said the quake was 11 miles deep.