PADANG, Indonesia (CNN) -- As many as 4,000 people could be buried under the rubble in the aftermath of the devastating earthquakes in Indonesia, United Nations officials said Saturday.

Indonesian soldiers crawl under the rubble of a collapsed building to search for victims.

Local disaster management officials put the death toll from the quakes at 540.

Earlier U.N. figures that put the death toll at 1,100 were just estimates, said El-Mostafa Benlamlih, a U.N. humanitarian coordinator.

A 7.6-magnitude quake struck Sumatra on Wednesday and a 6.6-magnitude earthquake hit Thursday morning in the same region.

Wednesday's quake reduced buildings to rubble in the city. People used hammers, chisels and bare hands to dig through debris for survivors and belongings. Watch CNN's Arwa Damon report on search-and-rescue efforts »

Staff at a local hospital treated the injured outside the semi-collapsed building as bodies of the dead lay in makeshift morgues.

An area that now looks like a flattened mess of destruction was, just days ago, a group of three villages.

Officials believe 90 percent of the residents -- as many as a few hundred people -- were buried, just one piece of the devastation from two large earthquakes that struck Indonesia in as many days.

The stench of dead bodies fills the air.

Indonesia's health ministry and ministry of social affairs said Friday they believe thousands remain buried beneath rubble.

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The West Sumatran capital, Padang, with about a million residents, is near the epicenter.

CNN's Arwa Damon spoke with a few dozen survivors from these villages in the area, most of whom only made it through because they weren't home during the quake. They remained huddled together in a tent, in shock over what had happened.

One older woman said eight of her family members were buried. She had been buried up to her chest and had to dig herself out.

Another survivor, a 27-year-old man, told CNN four of his family members were killed. His home used to be on top of a cliff in the area. Now, there is only mud.

Search and rescue teams are working with the military, but so far, only 25 bodies have been recovered.

With each passing day, the scope of the devastation grows. Watch aftermath at house leveled by quake »

President Obama spoke with Indonesian President Susilo Bambang Yudhoyono on Friday after trying several times to reach him, the White House said.

Obama received an update on the situation on the ground from Yudhoyono in the five-minute call.

The White House statement says Obama "offered, on behalf of the United States, to do everything we can to help alleviate the suffering and provide assistance to the relief operation."

The U.N.'s Holmes told reporters Thursday that hundreds are believed to be injured. "These numbers, I fear, will rise as more information becomes available," he added.

Telecommunications are difficult in the region, roads are cut off, and the hardest-hit area, including Padang, lacks power and other services, Holmes said. In addition, heavy rainfall has hindered search and rescue efforts.

Some have suggested the damage may be worse than that of a 6.3-magnitude quake centered in the central Java city of Yogyakarta in May 2006, Holmes said. That temblor killed more than 5,000 people and triggered fears of an eruption from a nearby volcano.

Many people wandered the streets of Padang stunned and dazed. Some searched the rubble for survivors.

Staff at a local hospital treated the injured outside the semi-collapsed building as bodies of the dead lay in makeshift morgues.

Several of the hospital's buildings were severely damaged. Damage in the town itself was spotty; some buildings remained intact near others in ruins.

"Aftershocks can be just as devastating as the initial quake," said Adjie Fachrurrazi, emergency response coordinator in Indonesia for the CARE aid organization, in a statement. "After an earthquake of this size, we know the immediate needs are going to be getting safe water, food and emergency supplies to the survivors. The question now is: How bad is it? We're hoping for the best, but the information so far is not looking good."

Amelia Merrick, the operations director for World Vision Indonesia, described the situation as "quite devastating."

"Bridges have gone down, phone lines are in total disrepair," she said. "It's difficult for us to assess the situation."

Earlier this month, an earthquake in West Java killed 57 people.

CNN's Arwa Damon and Ben Adams near Padang, Indonesia; Andy Saputra in Jakarta, Indonesia; and Tricia Escobedo in Atlanta, Georgia, contributed to this report.

All About Earthquakes • Indonesia • United Nations