Rescue teams are racing to reach thousands of people trapped under collapsed buildings after an earthquake struck off the Indonesian island of Sumatra.
Officials say up to 200 are dead but Siti Fadillah Supari, the Indonesian health minister, said on Thursday that given the widespread damage the number of victims "could be more ... than thousands".
The 7.6 magnitude undersea quake hit on Wednesday afternoon, about 50km from the coastal city of Padang with a population of 900,000, which bore the brunt of the temblor.
A second quake, with a 6.8 magnitude, struck Sumatra on Thursday morning, about 225km southeast of Padang, according to the US Geological Survey.
There have not yet been any reports of deaths or damage from the second quake, but the Indonesian geophysics and meteorology agency said the quake struck near several heavily populated towns.
Raphael Abreu, a geophysicist at the national earthquake information centre of the US Geological Survey in Colorado, told Al Jazeera that the second quake was "definitely capable of creating, by itself, significant damage to structures and property".
"And if combined with weakened situation in places like Padang which is still suffering the effects of the earlier quake, it could definitely compound the already significant damage in Padang."
Susilo Bambang Yudhoyono, Indonesia's president, instructed officials to "flood" Padang with aid and medical relief.
But Al Jazeera's Step Vaessen, reporting from Indonesia, said disaster teams were having trouble getting aid in.
She added that scientists had been warning of a major earthquake in the area for a long time, but Jakarta said it did not have funds for disaster preparation measures.
Malka Older, a programme director with aid agency Mercy Corp, based in Jakarta, said it has been extremely difficult to access the hardest-hit areas.
"We have a team on the ground as part of our ongoing programme but communication is down and we haven’t been able to get in contact with them, so we're sending additional people out there today to try and reach some of the damaged area.
"From what we know the government is responding quite quickly but the situation is making very difficult to get in there, for the government as well as NGOs, due to the breakdown in communications and infrastructure."
Three military transport planes were preparing to deliver aid including tents, blankets and medicine, disaster management agency spokesman Priyadi Kardono said.
"The effects of the earthquake could be as big as the Yogyakarta quake," he said, referring to a 2006 quake that killed more than 5,000 people and damaged or destroyed 150,000 homes.
'Big buildings down'
Officials in Padang said about 500 houses had caved in and witnesses said many buildings had collapsed after Wednesday's quake.
"The big buildings are down. The concrete buildings are all down, the hospitals, the main markets, down and burned. A lot of people died in there. A lot of places are burning," Jane Liddon, an Australian businesswoman, told Australian radio from Padang.
Padang's airport had to be closed after part of the roof collapsed and bridges and telecommunications links were also cut.
Rustam Pakaya, the head of the Indonesian health ministry's disaster centre, said that a hospital was among the buildings that had been destroyed.
He said a field hospital was being prepared to assist the injured and medical teams were on their way from neighbouring provinces.
"The earthquake was very strong," a resident named Kasmiati, who lives on the coast near to the epicentre, said.
"People ran to high ground. Houses and buildings were badly damaged," she said. "I was outside, so I am safe, but my children at home were injured."
Panicked residents rushed from their homes on to the streets after Wednesday's quake struck off Sumatra's west coast at 5:16pm (10:16 GMT).
It was felt in the Indonesian capital Jakarta, 940km away, and sent frightened office workers streaming out of buildings in Singapore as well as Malaysia's capital, Kuala Lumpur.
Dozens of aftershocks followed and the Pacific Tsunami Warning Centre issued a regional tsunami warning, but cancelled it afterwards.
The Indonesian earthquake came just hours after a series of tsunamis on the Pacific island nations of American Samoa and Samoa left more than 100 people dead.
Padang, the capital of Indonesia's West Sumatra province, sits on one of the world's most active fault lines along the so-called "Ring of Fire", the same one that cracked off Aceh, at the northern tip of Sumatra, in 2004 to trigger the Indian ocean tsunami that killed more than 220,000 people.
Padang was badly hit by an 8.4 magnitude quake in September 2007, when dozens of people died and several large buildings collapsed.
And geologists say the low-lying city is vulnerable to more quakes and tsunamis.
"There are three big volcanoes in West Sumatra - Merapi, Talang and Tandikat. We fear that this quake might cause volcanic eruptions there," said Surono, the head of the Geological Disaster Mitigation and Volcanology Centre in Indonesia.