PORT-AU-PRINCE, Haiti — Dazed survivors wandered past dead bodies in rubble-strewn streets Wednesday, crying for loved ones, and rescuers desperately searched collapsed buildings as fear rose that the death toll from Haiti's devastating earthquake could reach into the tens of thousands.

The first cargo planes with food, water, medical supplies, shelter and sniffer dogs headed to the Western Hemisphere's poorest nation a day after the magnitude-7 quake flattened much of the capital of 2 million people.

U.S. President Barack Obama said that one of the government's top priorities is to quickly locate U.S. Embassy employees and their families, as well as all other American citizens living and working in Haiti. He urged Americans trying to locate family members to contact the State Department at 1-888-407-4747.

Obama sought to show a swift and united disaster response with the U.S. as an assertive leader, but he said the effort must be an international one. "We are reminded of the common humanity that we all share," he said.

The president outlined a series of steps to help the Haitian people and said the U.S. commitment to its hemispheric neighbor will be unwavering. "We have to be there for them in their hour of need," the president said.

Some of the biggest immediate health threats for quake survivors include respiratory disease from inhaling dust from collapsed buildings and diarrhea from drinking contaminated water.

Survivors also will likely face an increased risk of dengue fever, malaria and measles — problems that plagued the impoverished country before, said Kimberley Shoaf, associate director of the UCLA Center for Public Health and Disasters.

The international Red Cross said a third of the country's 9 million people may need emergency aid, a burden that would test any nation and a crushing catastrophe for impoverished Haiti.

The U.S. Embassy had no confirmed reports of deaths among the estimated 40,000 to 45,000 Americans who live in Haiti, but many were struggling to find a way out of the country.

Tuesday's earthquake brought down buildings great and small — from shacks in shantytowns to President Rene Preval's gleaming white National Palace, where a dome tilted ominously above the manicured grounds.

Hospitals, schools and the main prison collapsed. The capital's Roman Catholic archbishop was killed when his office and the main cathedral fell. The head of the U.N. peacekeeping mission was missing in the ruins of the organization's multistory headquarters.

Police officers turned their pickup trucks into ambulances to carry the injured. Wisnel Occilus, a 24-year-old student, was wedged between two other survivors in a truck bed headed to a police station. He was in an English class when the earth shook at 4:53 p.m. and the building collapsed.

"The professor is dead. Some of the students are dead, too," said Occilus, who suspected he had several broken bones. "Everything hurts."

Other survivors carried injured to hospitals in wheelbarrows and on stretchers fashioned from doors.

In Petionville, next to the capital, people used sledgehammers and their bare hands to dig through a collapsed shopping center, tossing aside mattresses and office supplies. More than a dozen cars were entombed, including a U.N. truck.

Nearby, about 200 survivors, including many children, huddled in a theater parking lot using sheets to rig makeshift tents and shield themselves from the sun in 90-degree heat.

At a triage center improvised in a hotel parking lot, people with cuts, broken bones and crushed ribs moaned under tent-like covers fashioned from bloody sheets.

"I can't take it anymore. My back hurts too much," said Alex Georges, 28, who was still waiting for treatment a day after his school collapsed and killed 11 classmates. A body lay a few feet away.

"This is much worse than a hurricane," said doctors' assistant Jimitre Coquillon. "There's no water. There's nothing. Thirsty people are going to die."

If there were any organized efforts to distribute food or water, they were not visible.

The aid group Doctors Without Borders treated wounded at two hospitals that withstood the quake and set up tent clinics elsewhere to replace its damaged facilities. Cuba, which already had hundreds of doctors in Haiti, treated injured in field hospitals.

Bodies were everywhere in Port-au-Prince: those of tiny children adjacent to schools; women in the rubble-strewn streets with stunned expressions frozen on their faces; men hidden beneath plastic tarps and cotton sheets.

Haiti's leaders struggled to comprehend the extent of the catastrophe — the worst earthquake to hit the country in 200 years — even as aftershocks reverberated.

"It's incredible," Preval told CNN. "A lot of houses destroyed, hospitals, schools, personal homes. A lot of people in the street dead. … I'm still looking to understand the magnitude of the event and how to manage."

Preval said thousands of people were probably killed. Leading Sen. Youri Latortue told The Associated Press that 500,000 could be dead, but conceded that nobody really knows.

"Let's say that it's too early to give a number," Preval said.

As dusk fell, thousands of people gathered on blankets outside the crumpled presidential palace, including hundreds of women who waved their hands and sang hymns in a joyful, even defiant tone.

Ricardo Dervil, 29, said he decided to join the crowd because he was worried about aftershocks and was tired of seeing dead bodies.

"I was listening to the radio and they were saying to stay away from buildings," he said. "All I was doing was walking the street and seeing dead people."

Balancing suitcases and belongings on their heads, people streamed on foot into the Haitian countryside, where wooden and cinderblock shacks showed little sign of damage. Ambulances and U.N. trucks raced in the opposite direction, toward Port-au-Prince.

About 3,000 police and international peacekeepers cleared debris, directed traffic and maintained security in the capital. But law enforcement was stretched thin even before the quake and would be ill-equipped to deal with major unrest.

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Looting began immediately after the quake, with people seen carrying food from collapsed buildings. Many lugged what they could salvage and stacked it around them as they slept in streets and parks.

Obama promised an all-out rescue and humanitarian effort including the military and civilian emergency teams from across the U.S. Late Wednesday, the Navy said the amphibious assault ship USS Bataan had been ordered to sail as soon as possible with a 2,000-member Marine unit to join other warships headed to the Caribbean nation.

"We have to be there for them in their hour of need," Obama said. The first C-130 plane carrying part of a U.S. military assessment team arrived in Haiti, the U.S. Southern Command said..

Contributing: Mike Melia, Jennifer Kay, Edith M. Lederer, Frank Jordans, Bradley S. Klapper, Matthew Lee, Julie Pace, Jamey Keaton, Tales Azzoni, Alicia Chang and Andrea Rodriguez

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