Quake death toll passes 500

But that number was likely to soar, officials said, as the first outside rescue teams from the Indonesian army and health ministry reached the city to reinforce overwhelmed police on the ground.

Second quake hits Sumatra

Earlier today, a 6.8-magnitude earthquake struck Indonesia's Sumatra island again.

The second quake hit on land at 8:52am (1152 AEST), 225 kilometres southeast of the quake-hit city of Padang.

The United States Geological Survey put the quake at 6.8-magnitude.

"There are several heavily populated towns (in the area) there but no immediate reports of casualties," Indonesian geophysics and meteorology agency technical head Suharjono told AFP.

Race against time

Rescue workers are racing against time to assist victims.

Wednesday afternoon's 7.6-magnitude quake caused buildings to crumble and fires to rage in Padang, home to nearly a million people on the coast of Sumatra island. Communications and power remained cut off overnight.

"It's difficult to confirm the death toll as communications have been cut off," Disaster Management Agency spokesman Priyadi Kardono said in Jakarta earlier today.

"We need heavy machinery to lift the rubble... we expect that to arrive at the location soon," he added.

Late on Wednesday, Vice President Jusuf Kalla said: "People are trapped, hotels have collapsed, schools have collapsed, houses have collapsed and electricity has been cut off."

President Susilo Bambang Yudhoyono, briefing reporters in Tokyo as he returned from the G20 summit in the United States, urged officials to "flood" the city with aid and medical relief.

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 6,000.

British-based charity Oxfam said it had already earmarked STG200,000 ($A361,760) for relief efforts, including the distribution of emergency shelters, hygiene kits and clothing.

"We had aid ready because this area of Indonesia is susceptible to this type of tragedy," said Jane Cocking, the organisation's humanitarian director.

Local media reported that panicked residents rushed from their homes after the quake struck off Sumatra's west coast at 5:16pm on Wednesday (2016 AEST), 47 kilometres northwest of Padang. Dozens of aftershocks followed.

The Pacific Tsunami Warning Center in Hawaii initially put out a tsunami watch but later withdrew it.

The tremor came not long after a massive quake measuring 8.0 in magnitude spawned a deadly tsunami in the Samoan islands of the South Pacific.

Like Indonesia, the islands sit on the volatile "Ring of Fire", a massive zone of volcanic instability that encircles the Pacific.

The Padang quake was felt in the Indonesian capital Jakarta, 940 kilometres away, and sent frightened office workers streaming out of buildings in nearby Singapore and the Malaysian capital Kuala Lumpur.

"The shaking was the worst I had ever felt," Yuliarni, a resident of Pariaman district outside Padang told TVOne news channel.

"Houses have collapsed, the lights and electricity were cut off... People were fleeing to higher ground and some were hurt," she said.

The quake caused a landslide that destroyed houses at Lake Maninjau, inland from Padang, local resident Hafiz told the channel, while the city airport was slightly damaged but was expected to reopen early Thursday.

Geologists said Padang, which lies near the colliding Indo-Australian and Eurasian tectonic plates, was 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," Geological Disaster Mitigation and Volcanology Centre head Surono told AFP.

Padang lies on the same tectonic faultline 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.