ADANA, Turkey — A powerful 7.8 magnitude earthquake rocked wide swaths of Turkey and neighboring Syria on Monday, killing more than 3,400 people and injuring thousands more as it toppled thousands of buildings and trapped residents under mounds of rubble.

Authorities feared the death toll would keep climbing as rescuers searched through tangles of metal and concrete for survivors in a region beset by Syria's 12-year civil war and a refugee crisis.

Residents jolted out of sleep by the predawn quake rushed outside in the rain and snow to escape falling debris, while those who were trapped cried for help. Throughout the day, major aftershocks rattled the region, including a jolt nearly as strong as the initial quake. After night fell, workers were still sawing away slabs and pulling out bodies as desperate families waited for news on trapped loved ones.

“My grandson is 1 1/2 years old. Please help them, please. We can't hear them or get any news from them since morning. Please, they were on the 12th floor,” Imran Bahur wept by her destroyed apartment building in the Turkish city of Adana. Her daughter and family were still not found.

Tens of thousands who were left homeless in Turkey and Syria faced a night in the cold. In Turkey's Gaziantep, a provincial capital about 33 kilometers from the epicenter, people took refuge in shopping malls, stadiums and community centers. Mosques around the region were opened to provide shelter.

Turkish President Recep Tayyip Erdogan declared seven days of national mourning.

The quake, which was centered on Turkey's southeastern province of Kahramanmaras, sent residents of Damascus and Beirut rushing into the street and was felt as far away as Cairo.

The quake piled more misery on a region that has seen tremendous suffering over the past decade. On the Syrian side, the area 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 the civil war.

In the rebel-held enclave, hundreds of families remained trapped in rubble, the opposition emergency organization, called the White Helmets said in a statement. The area is packed with some 4 million people displaced from other parts of the country by the war. Many of them live in buildings that are already wrecked from past bombardments.

Strained health facilities quickly filled with injured, rescue workers said. Others had to be emptied, including a maternity hospital, according to the SAMS medical organization.

More than 7,800 people were rescued across 10 provinces, according to Orhan Tatar, an official with Turkey's disaster management authority.

The region sits on top of major fault lines and is frequently shaken by earthquakes. Some 18,000 were killed in similarly powerful earthquakes that hit northwest Turkey in 1999.

The U.S. Geological Survey measured Monday's quake at 7.8, with a depth of 18 kilometers (11 miles). Hours later, a 7.5 magnitude temblor struck more than 100 kilometers (60 miles) away.

The second jolt in the afternoon caused a multistory apartment building to topple face-forward onto the street in the Turkish city of Sanliurfa. The structure disintegrated into rubble and raised a cloud of dust as bystanders screamed, according to video of the scene.

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 330 kilometers (200 miles) to the northeast.

In Turkey alone, more than 5,600 buildings were destroyed, authorities said. Hospitals were damaged, and one collapsed in the Turkish city of Iskenderun.

Bitterly cold temperatures could reduce the time frame that rescuers have to save trapped survivors, said Dr. Steven Godby, an expert in natural hazards at Nottingham Trent University. The difficulty of working in areas beset by civil war would further complicate rescue efforts, he said.

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. The vast majority were for Turkey, with Russian and even an Israeli promise of help to the Syrian government, but it was not clear if any would go to the devastated rebel-held pocket in the northwest.

The opposition's Syrian Civil Defense described the situation in the enclave as “disastrous.”

The opposition-held area, centered on the province of Idlib, has been under siege for years, with frequent Russian and government airstrikes. The territory depends on a flow of aid from nearby Turkey for everything from food to medical supplies.

At a hospital in Idlib, Osama Abdel Hamid said most of his neighbors died. He said their shared four-story building collapsed just as he, his wife and three children ran toward the exit. A wooden door fell on them and acted as a shield.

“God gave me a new lease on life,” he said.

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.

Television 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. Turkish broadcaster CNN Turk said a woman was pulled out alive in Gaziantep after a rescue dog detected her.

In Adana, 20 or so people, some in emergency rescue jackets, used power saws atop the concrete mountain of a collapsed building to saw out space for any survivors to climb out or be rescued.

“I don't have the strength anymore,” one survivor could be heard calling out from beneath the rubble of another building in Adana earlier in the day, as rescue workers tried to reach him, said a resident, journalism student Muhammet Fatih Yavuz.

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 excavators dug through the rubble below.

At least 2,316 people were killed in 10 Turkish provinces, with more than 13,000 injured, according to Turkish authorities. The death toll in government-held areas of Syria climbed to 656 people, with some 1,400 injured, according to the Health Ministry. In the country's rebel-held northwest, groups that operate there said the death toll was at least 450, with many hundreds injured.

Huseyin Yayman, a legislator from Turkey's Hatay province, said several of his family members were stuck under the rubble of their collapsed homes.

“There are so many other people who are also trapped,” he told HaberTurk television by phone. “There are so many buildings that have been damaged. People are on the streets. It's raining, it's winter.”