Rescuers in Turkey and Syria dug through the freezing night hunting for survivors among the rubble of thousands of buildings felled in a series of violent earthquakes. The confirmed death toll across the two countries has soared above 4,300 after a swarm of strong tremors near the Turkey-Syria border - the largest of which measured at a massive 7.8-magnitude.
The head of Syria's National Earthquake Centre, Raed Ahmed, told pro- government radio that this was "historically, the biggest earthquake recorded in the history of the centre".
Rescue efforts were being hampered by a winter blizzard that covered major roads in ice and snow.
Television images showed shocked people in Turkey standing in the snow in their pajamas, watching rescuers dig through the debris of damaged homes. The first quake struck at 04:17 am local time (01:17 am GMT) at a depth of about 17.9 kilometres (11 miles) near the Turkish city of Gaziantep, which is home to around two million people, the US Geological Survey said.
Turkish and Syrian disaster response teams report more than 5,600 buildings have been flattened across several cities, including many multi-storey apartment blocks that were filled with sleeping residents when the first quake struck.
Turkey's AFAD emergencies service centre put the first quake's magnitude at 7.4, adding that it was followed by more than 40 aftershocks. Turkish President Recep Tayyip Erdogan conveyed his sympathies and urged national unity.
"We hope that we will get through this disaster together as soon as possible and with the least damage," the Turkish leader tweeted.
US National Security Advisor Jake Sullivan said Washington was "profoundly concerned".
"We stand ready to provide any and all needed assistance," Sullivan said.
In Gaziantep, a Turkish city home to countless refugees from Syria's decade-old civil war, rescuers picking through the rubble screamed, cried and clamoured for safety as another building collapsed nearby without warning.
The initial earthquake was so large it was felt as far away as Greenland, and the impact is big enough to have sparked a global response.
The Syrian health ministry reported damage across the provinces of Aleppo, Latakia, Hama and Tartus, where Russia is leasing a naval facility. AFP correspondents in northern Syria said terrified residents ran out of their homes after the ground shook.
Even before the tragedy, buildings in Aleppo, Syria's pre-war commercial hub, often collapsed due to the dilapidated infrastructure after more than a decade of war as well as little oversight to ensure safety of new construction projects, some built illegally.
Naci Gorur, an earthquake expert with Turkey's Academy of Sciences, urged local officials to immediately check the region's dams for cracks to avert potentially catastrophic flooding.
Turkey is in one of the world's most active earthquake zones. The Turkish region of Duzce suffered a 7.4-magnitude earthquake in 1999 - the worst to hit Turkey in decades.
That quake killed more than 17,000 people, including about 1,000 in Istanbul. Experts have long warned a large quake could devastate Istanbul, which has allowed widespread building without safety precautions.