There is no official estimate on the number of deaths and informal estimates vary. The International Red Cross said Thursday the 7-magnitude earthquake likely killed 45,000 to 50,000 people, but U.N. Secretary-General Ban Ki-moon said it may be days before an "educated guess" could be made.
Aid workers told CNN they were met by the smell of dead bodies when they entered Port-au-Prince, Haiti's capital, to aid in search-and-rescue missions or to provide assistance two days after the earthquake struck.
Rubble-covered roads, downed trees, spotty communication and lack of electricity hampered efforts to get supplies to victims, aid officials said.
The U.S. military opened the Port-au-Prince airport and Coast Guard and Navy vessels are operating off the coast, a Pentagon spokesman said in a release. The 82nd Airborne Division's 2nd Brigade Combat Team has begun moving into the country to help provide security. The aircraft carrier USS Carl Vinson and hospital ship USNS Comfort also will be deployed to Haiti.
"This is a significant effort that is first about saving lives in the critical first 72-hour period, and then laying the groundwork for providing access to critically needed commodities -- medical services, food, water, water purification -- and then laying the groundwork for rebuilding," said Rajiv Shah, director of the U.S. Agency for International Development.
Makeshift morgues sprang up around the Haitian capital, as bloated bodies piled up, several media outlets reported.
Overnight, tens of thousands of Haitians slept on sidewalks in the capital or wandered the streets, The Washington Post reported. In one now-flattened neighborhood, streets and sidewalks were crammed with people sleeping on mattresses, plastic chairs and bits of cardboard, witnesses reported.
"The few houses still standing have cracks. Everyone thinks they are too dangerous," one man told the Post.
"I am sure that I will not sleep in the street tonight. But I am sure a lot of people will sleep in the street tonight," Haitian President Rene Preval said on NBC's "Today" show.
Dozens of nations and a number of international non-government organizations sprung to action, Voice of America reported. But relief officials said the country's post-earthquake conditions hampered efforts to transport aid to victims.
The International Committee of the Red Cross set up a Web site to help people get in touch with friends or relatives with whom they've lost contact in Haiti.
The United Nations announced $10 million in emergency aid and the World Bank pledged $100 million, CNN reported. The European Union pledged $4.4 million, the Times said. The United States would provide $100 million, President Barack Obama said Thursday.