At least 1,100 people have died in the earthquake that struck the Indonesian island of Sumatra on Wednesday, the UN humanitarian chief has said.
John Holmes said many hundreds more had been injured, and both figures were set to rise further.
Rescuers are working into the night to find survivors in the rubble of hundreds of collapsed buildings.
The 7.6-magnitude quake struck close to the city of Padang, the capital of West Sumatra province.
The earthquake brought down hospitals, schools and shopping malls, cut power lines and triggered landslides.
AT THE SCENE
Karishma Vaswani, BBC News, Padang
As every hour passes, the scale of this disaster becomes all too clear. Dozens of homes and office buildings have collapsed. A restaurant frequented by college students is now a pile of rubble. Officials say at least 60 people are trapped underneath.
Scenes like this are repeated all across Padang. Rescue efforts have been hampered by poor communication lines and bad roads.
The first flights carrying food and aid arrived this morning but it's nowhere near enough. Hospitals in Padang have been severely damaged in this disaster.
People are being treated in makeshift tents. There are still many parts of the city that rescue workers haven't been able to reach which is raising fears the number of dead will almost certainly rise.
Indonesian President Susilo Bambang Yudhoyono visited some of the worst-hit areas.
"I ask rescue workers to continue working in teams with clear goals to keep looking for survivors...," he said.
"This is a natural disaster, so let us remain strong in dealing with it."
Meanwhile US President Barack Obama, who spent part of his childhood in Indonesia, said he was "deeply moved" by the suffering caused by the quake.
"Indonesia is an extraordinary country who has known extraordinary hardships from natural disasters. I know that the Indonesian people are strong and resilient and have the heart to overcome this challenge," he said.
UN Under-Secretary General for Humanitarian Affairs John Holmes told reporters: "The latest figures we have suggest the death toll has risen already to 1,100.
"Obviously [there are] many hundreds of injured people as well, and again these numbers, I fear, will rise as more information becomes available."
An assessment team is to arrive in Padang on Friday, and UN officials will decide whether to launch an emergency appeal or take money from the organisation's Central Emergency Relief Fund, Mr Holmes added.
Indonesian health officials have already predicted thousands of deaths, comparing the quake to one in the Javan city of Yogyakarta in 2006.
A second quake of 6.8 struck close to Padang at 0852 local time (0152 GMT) on Thursday but there were no immediate reports of casualties or damage.
The first earthquake struck at 1716 local time (1016 GMT) on Wednesday, some 85km (55 miles) under the sea, north-west of Padang, the US Geological Survey said.
One of the worst disasters appeared to be the collapse of a school in Padang.
One mother, Andriana, told AFP news agency she had been at the school since the first quake occurred, hoping for news of her 14-year-old daughter.
"I haven't been home yet and keep praying to God my daughter is alive."
Police said nine children had been found alive but that eight bodies had also been pulled from the rubble so far.
Rescuers and medical workers are struggling to cope with the amount of destruction and the sheer number of victims.
Titi Moektijasih, of the UN Office of Coordination of Humanitarian Affairs, told AFP that emergency efforts so far were insufficient.
PADANG: KEY FACTS
Population of 900,000, capital of West Sumatra province
On coastal plain, surrounded by mountains inland
Lies on one of world's most active fault lines
Near major quake epicentres in March 2007 and April 2005
"Compared to the extent of the damage, you see there should be more equipment, more people to do this."
David Lange, a doctor with Surfaid International, told the BBC one of the hospitals was "completely destroyed" and medical workers were struggling to cope.
"They are trying to operate in the parking lot, in a tent, in the mud."
Bob McKerrow, Red Cross head of operations in Indonesia, told the BBC it had more than 400 personnel on the ground, including 50 doctors flown in on Thursday morning.
"But it's just such a vast area to be working in with such bad infrastructure," he said. "I mean the roads and bridges have all been damaged, so [there is] a challenge ahead of us."
The quake brought down telephone lines, severely affecting communications with the region and making it difficult to assess the scale of the damage. Power has now been restored to some parts of Padang.
Health ministry teams and Indonesian soldiers have arrived in the city to aid the search for survivors. A shortage of heavy machinery remains a problem.
MAJOR INDONESIAN QUAKES
26 Dec 2004: Asian tsunami kills 170,000 in Indonesia alone
28 March 2005: About 1,300 killed after a magnitude 8.7 quake hits the coast of Sumatra
27 May 2006: Quake hits ancient city of Yogyakarta, killing 5,000
17 July 2006: A tsunami after a 7.7 magnitude quake in West Java province kills 550 people
30 Sept 2009: 7.6 magnitude quake near Sumatran city of Padang, thousands feared dead
1 Oct 2009: Second of two quakes near Padang, magnitude 6.8 - no damage or casualties reported
Food, medicine and body bags have begun to arrive. Tents and blankets are also on their way.
Wednesday's quake struck about 600km north-west of Padang, along the same fault line that spawned the 2004 Asian tsunami that killed more than 230,000 people in a dozen countries.
Geologists have long warned that Padang - a city of 900,000 people - could one day be completely destroyed by an earthquake because of its location.
The earthquake struck nearly 12 hours after a powerful quake in the South Pacific that triggered a devastating tsunami but experts said the two events were unrelated.
"They were 10,000km (6,200 miles) apart," New Zealand seismologist Bill Fry told AFP news agency.
"You can get quakes that are close temporally and spatially as one transfers stress to another place against the fault, but that's not possible this far apart."
Australia is among the countries that have offered to send emergency assistance to Indonesia if needed.