The death toll climbed and aftershocks continued a day after a powerful earthquake struck the Nepal region, triggering an avalanche on Mount Everest and setting off an urgent aid response.

The key information known at this time:

___

CASUALTIES AND DAMAGE

Indications suggest the death toll across four countries is likely to rise substantially in the coming days. The magnitude-7.8 quake was the worst to hit Nepal in eight decades and caused damage and fatalities in neighboring countries. In addition to more than 2,100 people killed in Nepal, a total of at least 61 died in India, Bangladesh, China's region of Tibet and Pakistan.

The earthquake hit a heavily populated area of Nepal, including the capital, Kathmandu — where at least 721 people died — and its impact spread far beyond the Kathmandu Valley. Strong aftershocks were still being felt Sunday, including one with a magnitude of 6.7.

Local hospitals were filling with injured residents, and Kathmandu's international airport was shut down for hours, hampering initial relief efforts in the isolated mountainous country.

Among the destroyed buildings in Kathmandu was the nine-story Dharahara Tower, a landmark built by Nepal's royal rulers as a watchtower in the 1800s and a UNESCO-recognized historical monument. It was reduced to rubble and there were reports of people trapped underneath.

___

AID EFFORTS

With Kathmandu's airport reopened Sunday, the first aid flights began delivering supplies. The first to respond were Nepal's neighbors India, China and Pakistan.

Indian air force planes landed with 43 tons of relief material, including tents and food, and nearly 200 rescuers, India's External Affairs Ministry spokesman Vikas Swarup said. The planes were returning to New Delhi with Indian nationals stranded in Kathmandu. More aid flights were planned for Sunday.

A 62-member Chinese search and rescue team also arrived Sunday. Other countries sending support included the United Arab Emirates, Germany and France.

Pakistan prepared to send four C-130 aircraft, carrying a 30-bed temporary hospital comprising army doctors, surgeons and specialists. An urban search and rescue team was also sent with ground-penetrating radars, concrete cutters and sniffing dogs. Pakistan was also sending 2,000 ready-to-eat meal packs, water bottles, medicines, 200 tents, 600 blankets and other necessary items.

___

MOUNT EVEREST SITUATION

A devastating avalanche swept across Mount Everest after the quake, killing at least 17 people and injuring dozens.

The first group of survivors from the avalanche were flown to Kathmandu on Sunday and taken to hospitals. None appeared to have life-threatening injuries.

The avalanche struck near one of the famed mountain's most dangerous spots. It swept down between the Khumbu Icefall, known for its harsh conditions, and the base camp used by international climbing expeditions.

There were unverified reports of avalanches on other parts of the mountain.

Facebook postings by climbers suggested that some people may have been buried in their tents when the avalanche hit. Climbers and their support teams were leaving the base camp Saturday looking for safer locations.

___

WHAT HAPPENED? AND WHAT HAPPENS NEXT?

U.S. Geological Survey seismologist Paul Earle said the quake happened on what is known as a "thrust fault." That describes the situation when one piece of the Earth's crust is moving beneath another piece.

In this case, it's the Indian plate that is moving north at 45 millimeters (1.7 inches) a year under the Eurasian plate to the north, Earle said. It's a different type of earthquake than the one that caused the 1906 San Francisco earthquake.

"This is what builds the Himalayan mountain range," Earle said.

The region and particular fault has a history of damaging earthquakes, including four temblors with magnitudes greater than 6.0 in the past 100 years, Earle said, warning that landslides are a particular worry now, given the steep slopes in the region.

___

HOW PREPARED IS NEPAL TO COPE WITH SUCH A DISASTER?

Nepal is a relatively poor country without extensive resources despites its rich cultural heritage and spectacular mountain scenery.

It has been plagued by instability in recent years, and general strikes have recently brought chaotic scenes to Kathmandu.

Nepal's constitution was supposed to have been written by the Constituent Assembly that was elected in 2008, following the end of a 10-year Maoist insurgency and the overthrow of the centuries-old monarchy, but the assembly was hampered by infighting and never finished its work.

The current assembly was chosen in 2013, but has faced the same problem.

Neighboring Pakistan has offered help, and an international aid effort is likely to begin once Kathmandu's international airport can be reopened.

___

Associated Press writer Seth Borenstein in Washington contributed to this report.