Foreign medical teams are working feverishly to stop countless more dying from massive injuries in Haiti, as an aftershock measuring 4.8 on the Richter scale hits the already devastated nation.
The USGS said the aftershock hit at 1654 GMT on Thursday (0354 AEDT on Friday) and was centered 40 kilometres west-southwest of Port-au-Prince, with a depth of about 10 kilometres.
The aftershock followed a heavier 6.1-magnitude jolt early on Wednesday, after the catastrophic 7.0-magnitude quake nine days ago which killed up to 200,000 people and left an estimated one million homeless.
US troops on Thursday were also moving to clear the main port in the devastated capital hoping to re-open it on Friday and start a massive distribution of aid across the Caribbean nation.
Rescue teams from around the world were also still combing the debris for survivors in and around Port-au-Prince, after two children were miraculously found alive on Wednesday.
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But those pulled from the rubble still face a desperate fight for life.
Tens of thousands remain seriously wounded in makeshift field hospitals set up around the ravaged capital, and gangrene has already begun to eat its way through many wounds in the tropical heat.
Doctors, working in miserable conditions lacking supplies and modern equipment, have already carried out countless amputations to save victims with serious crush wounds or to repair internal injuries.
"We carried out 30 operations on Tuesday. In the previous six days we have cared for more than 1,000 Haitians," said French doctor Thierry Allafort Duverger at a clinic in what used to be the posh suburb of Petionville.
A 1,000-bed capacity US naval hospital ship is also off Haiti with about 800 medical personnel, and has begun taking on board the worst of the injured.
In a bid to move an estimated 500,000 left homeless by the January 12 quake, the Haitian government said it was seeking to move them out of squalid, stinking tent cities into temporary housing.
The government was paying some 34 buses to take victims to the south and north of the country to hastily set up villages designed to hold 10,000 people each.
The International Organization for Migration estimated on Thursday that at least half a million people are now living outdoors in improvised camps, and warned the number was climbing.
"So far some 447 improvised settlements comprising at least 500,000 people have been identified in this city alone, out of which 350 settlements have been assessed by IOM, the Haitian government and humanitarian partners," the agency said in a statement.
Most people had cobbled together makeshift shelters from sheeting, blankets, cardboard or bits of debris, while some had received tents from Haitian authorities or the US forces.
The search for survivors was still going on, buoyed after a five-year-old boy was found in the wreckage of his home on Wednesday while neighbours dragged out an 11-year-old girl in another part of the city.
Mendji Bahina Sanon, 11, was taken to a French-run field hospital in the capital, clearly traumatised by her nightmarish ordeal, crying out: "Don't leave me Mama."
Asked when the search would be called off, Elisabeth Byrs, a spokeswoman for the UN Organization for the Coordination of Humanitarian Affairs (OCHA) said it was up to the authorities on the ground.
"As long as there is hope, rescue operations continue," she told AFP. "No trace is neglected and rescuers worked as much on the eighth day as on the first."
Thousands of US troops have poured into Haiti and other nations have pledged security forces to help distribute aid, provide medical treatment and try to keep the streets secure from looters and gangs.
In Leogane, west of Port-au-Prince, which was almost totally wrecked in the quake, the arrival of two truckloads of flour, wheat, sugar and beans sparked fights on Wednesday that delayed distribution.
International efforts are also focusing on rebuilding the country, with a major donor conference set for Monday in Montreal.
International Monetary Fund chief Dominique Strauss-Kahn called for a multilateral aid plan on the scale of the US "Marshall Plan" that rebuilt Europe after World War II.
The United States is sending in 4,000 extra troops, and a US general said Port-au-Prince port should be up and running again on Friday.
The US military said four airports were now being used to ferry in relief supplies, easing some of the pressure on the jammed international airport at Port-au-Prince.
Backed by three ships and a squadron of helicopters, all the troops should be in place by the weekend as the total US forces helping out in Haiti swells above 15,000 in coming days.
The United Nations, meanwhile, confirmed the number of UN staffers killed dead in quake has risen from 49 to 61, with nearly 180 others, mostly Haitians, unaccounted for.
The 7.0-magnitude quake marks the worst disaster in the UN's history.
