Haitian Consul General Felix Augustin said the capital Port-au-Prince "is flattened, CNN reported.
"More than 100,000 are dead," Felix Augustin told reporters.
Haitian Prime Minister Jean-Max Bellerive said it could be much worse, saying several hundred thousand people may have been killed.
"I hope that is not true, because I hope the people had the time to get out," Bellerive told the U.S. news network.
"Because we have so [many] people on the streets right now, we don't know exactly where they were living. But so many, so many buildings, so many neighborhoods totally destroyed, and some neighborhoods we don't even see people."
Rescue teams were picking their way through the rubble of collapsed buildings and an international full-court press was initiated to rush aid to the impoverished Caribbean island nation. The United Nations announcing it was airlifting 86 metric tons of food, enough for 500,000 emergency meals.
"We will work with the Haitian government, with our humanitarian partners on the ground, and with governments across the world as part of a coordinated international rescue and recovery effort," Josette Sheeran, head of the U.N. World Food Program, said in a statement.
The agency said the first priorities were finding survivors pinned under rubble, treating people with major injuries and providing clean water and sanitation.
Kristie van de Wetering, a former Oxfam charity employee in Port-au-Prince, told CNN the situation was "very chaotic" with many buildings wiped out.
"We can hear people calling for help from every corner," she said.
U.N. Secretary-General Ban Ki-moon said the extensive damage to buildings and infrastructure in Port-au-Prince means "there is no doubt that we are facing a major humanitarian emergency and that a major relief effort will be required." He called for the world to "come to Haiti's aid in this hour of need."
"Port-au-Prince is devastated, lot of deaths. SOS. SOS," Louise Ivers, clinical director of Partners in Health, told colleagues via e-mail. "Temporary field hospital by us ... needs supplies, pain meds, bandages. Please help us."
Tuesday's quake flattened the National Palace, numerous shantytown dwellings, hospitals and the U.N. mission, and crushed power and telephone lines, officials said.
The earthquake struck just before 5 p.m. Tuesday about 10 miles southwest of the Haitian capital, the U.S. Geological Survey said. Many aftershocks followed and more were expected.
The earthquake's force was felt across the border in the Dominican Republic on the eastern part of the island of Hispaniola. Officials said high-rise buildings Santo Domingo shook, sending people into the streets, The New York Times said.
CNN reported a spokesman for the International Federation of the Red Cross said up to 3 million people may have been affected.
Robert Poff, director of disaster services for the Salvation Army in Haiti, told the U.S. news network he saw buildings "pancaking" down.
"Traffic, of course, came to a standstill, while thousands of people poured out into the streets, crying, carrying bloody bodies, looking for anyone who could help them," Poff said. "We piled as many bodies (as possible) into the back of our truck, and took them down the hill with us, hoping to find medical attention."
U.S. President Barack Obama said there would be a "swift, coordinated and aggressive" U.S. response.
"The reports and images that we've seen of collapsed hospitals, crumbled homes and men and women carrying their injured neighbors through the streets are truly heart-wrenching," Obama said.