A 7.8-magnitude earthquake hit Turkey and Syria on Monday, killing more than 100 people.

Emergency service officials in Turkey put the initial death toll at 76, although it threatened to climb substantially higher because the night-time disaster had flattened dozens of apartment blocks across major cities.

According to the latest data, at least 245 people also died in parts of Syria, as well as the northern areas held by pro-Turkish factions, according to state media and a local hospital.

Television images showed shocked people in Turkey standing in the snow in their pajamas, watching rescuers dig through the debris of damaged homes.

The quake struck at 04:17 am local time (0117 GMT) at a depth of about 17.9 kilometers (11 miles), the US Geological Survey said, with a 6.7-magnitude aftershock striking 15 minutes later.

Turkey's AFAD emergencies service center put the first quake's magnitude at 7.4, adding that it was followed by dozens of aftershocks.

The earthquake was one of the most powerful to hit the region in at least a century, affecting southeastern parts of Turkey that are home to millions of refugees from Syria and other war-torn regions.

"There are many damaged buildings," AFAD chief Orhan Tatar said in nationally televised remarks. "People should avoid buildings."

Turkish President Recep Tayyip Erdogan, who will be under pressure to oversee an effective response to the disaster heading to a tightly-contested May 14 election, 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.

- 'Damage is serious' -

The earthquake leveled dozens of buildings across major cities of southern Turkey as well as neighboring Syria.

Images on Turkish television and social media showed rescuers digging through the rubble of leveled buildings in the city of Kahramanmaras and neighboring Gaziantep.

A fire lit up the night sky in one image from Kahramanmaras, although its origin remained unclear.

NTV television said buildings also crumbled in the cities of Adiyaman, Malatya and Diyarbakir.

Kahramanmaras Governor Omer Faruk Coskun said it was too early to estimate the death toll because so many buildings were destroyed.

"It is not possible to give the number of dead and injured at the moment because so many buildings have been destroyed," Coskun said. "The damage is serious."

A famous mosque dating back to the 13th century partially collapsed in the province of Maltaya.

In other cities, rescuers sounded anguished as they struggled to reach survivors trapped under the debris.

"We hear voices here -- and over there, too," one rescuer was overheard as saying on NTV television in front of a flattened building in the city of Diyarbakir.

"There may be 200 people under the rubble."

- 'Biggest earthquake' -

Syrian state television reported that a building near Latakia, on the west coast of Syria, had collapsed.

Government media said several buildings had partially collapsed in Hama, central Syria, with civil defense and firefighters working to pull survivors out of the rubble.

Raed Ahmed, who heads Syria's National Earthquake Center, told government radio that this was "historically, the biggest earthquake recorded in the history of the center".

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 floodings.

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.

A magnitude-6.8 quake hit Elazig in January 2020, killing more than 40 people.

And in October of that year, a magnitude-7.0 quake hit Turkey's Aegean coast, killing 114 people and wounding more than 1,000.