PADANG: Rescue workers used excavators Thursday to pull out victims, some screaming in pain, from the heavy rubble of buildings felled by a powerful earthquake that killed at least 531 people. The death toll was expected to rise.

The brunt of Wednesday's 7.6-magnitude earthquake, which originated in the sea off Sumatra island, appeared to have been borne by Padang town where 376 people were killed. Four other districts accounted for the remaining deaths.

The region was jolted by another powerful earthquake Thursday morning, causing damage but no reported fatalities.

More than 500 buildings including hotels, schools, hospitals and a mall were destroyed or damaged in Padang. Thousands of people were believed to be trapped in the rubble.

"Oh God, help me! help me!" Friska Yuniwati, a 30-year-old woman, screamed in pain, as she was carried to an ambulance in downtown Padang. She had been pulled out minutes earlier from the rubble of a house, her face covered in bruises and eyes shut.

Padang's state-run Djamil Hospital was overwhelmed by the influx of victims and families. Dozens of injured people were being treated under tents outside the hospital, which was itself partly damaged.

"Let's not underestimate (the disaster). Let's be prepared for the worst. We will do everything we can to help the victims," President Susilo Bambang Yudhoyono said in Jakarta before flying to Padang, a coastal city of 900,000 and capital of West Sumatra province.

A total of 531 people were confirmed dead and 440 were seriously injured, the Social Affairs Ministry's crisis center said. Thousands were believed trapped, said Rustam Pakaya, head of the Health Ministry's crisis center.

One focus for emergency workers was a collapsed 4-story concrete building in downtown Padang, where 30 children had been taking classes when the quake struck. Four students were found alive and six bodies were dug from the rubble. Dozens were missing, said Jamil, a volunteer. "It's getting very difficult now to find more victims," he said.

Parents of missing students stayed up all night, waiting for signs of life.

"My daughter's face keeps appearing in my eyes ... my mind. I cannot sleep, I'm waiting here to see her again," a woman who identified herself only as Imelda said, tears rolling down her face. She said her 12-year-old daughter Yolanda was in the school for science lessons.

"She is a good daughter and very smart. I really love her. Please, God help her," she said.

In another building, rescue workers passed a plastic bottle of water through an opening in the rubble to a person trapped underneath.

The president ordered the military to deploy emergency response teams from Jakarta, West Sumatra and North Sumatra provinces. He said the military will provide earth-moving equipment to clear the rubble.

SurfAid, a New Zealand-based medical aid group, said its program director David Lange narrowly escaped death when he fled the Ambacang Hotel minutes before it collapsed.

"People are trapped and screaming for help but they are below huge slabs which will take heavy equipment to move," Lange was quoted as saying in a statement by SurfAid.

"I saw dozens of the biggest buildings collapsed in town. Most of the damage is concentrated in the commercial center market, which was fully packed," he said.

At least 80 people were missing at the five-story Ambacang Hotel, said Indra, a paramedic who uses only one name.

Terrified residents who spent a restless night, many sleeping outdoors, were jolted by the new quake Thursday morning.

The U.S. Geological Survey said the quake hit about 150 miles (240 kilometers) south of Padang. It damaged 1,100 buildings, including mosques and homes, in the town of Jambi, according to Mayor Hasfiah, who uses only one name like many Indonesians. He said there were no deaths but dozens of people were injured.

The quake was so powerful that it caused buildings to sway hundreds of miles (kilometers) away in Malaysia and Singapore. In Padang, children screamed as thousands of frantic residents fled in cars and motorbikes, honking horns. They feared the quake would trigger a tsunami, but no giant waves struck.

The quake severed roads and cut off power and communications to Padang, and the extent of damage in surrounding areas was still unclear.

Indonesia, a poor, sprawling nation, sits on a major geological fault zone and is frequently hit by earthquakes. The latest quakes were along the same fault line that spawned the 2004 Asian tsunami that killed 230,000 people in a dozen nations.

Geologists said the Indonesia quakes were not related to another deadly quake Tuesday that hit islands in the South Pacific.

Padang's mayor appealed for assistance on Indonesian radio station el-Shinta.

"We are overwhelmed with victims and ... lack of clean water, electricity and telecommunications," Mayor Fauzi Bahar said. "We really need help. We call on people to come to Padang to evacuate bodies and help the injured."

Finance minister Sri Mulyani said the government has allocated $25 million for a two-month emergency response. She said the earthquake will seriously affect Indonesia's economic growth, because West Sumatra is a main producer of crude palm oil.

"This region has been damaged seriously, including its infrastructure," Mulyani said.