PORT-AU-PRINCE, Haiti - A powerful aftershock sent Haitians screaming into the streets on Wednesday, collapsing buildings, cracking roads and adding to the trauma of a nation stunned by an apocalyptic quake eight days ago.

The magnitude-5.9 jolt matched the strongest of the aftershocks that have followed the huge quake of Jan. 12 that devastated Haiti's capital.

The new temblor collapsed seven buildings in Petit-Goave, the seaside town closest to the epicentre, according to Mike Morton of the UN Disaster Assessment and Coordination agency, but there were no reports of people crushed or trapped, perhaps because the earlier quake frightened most people into sleeping outside.

Wails of terror erupted in Port-au-Prince, where the aftershock briefly interrupted rescue efforts amid the broken concrete of collapsed buildings, and prompted doctors and patients to flee the University Hospital.

Foreign Affairs Minister Lawrence Cannon reported minor damage to the roof and second floor of the Canadian Embassy in Port-au-Prince after the aftershocks. No one was injured at the embassy, where 56 Canadians had sought refuge.

Hundreds of thousands of Haitians remain homeless, hungry and in mourning - most still waiting for the benefits of a nearly US$1 billion global aid campaign that has brought hundreds of doctors and thousands of troops to the impoverished Caribbean nation.

UN humanitarian chief John Holmes told reporters in New York that two million Haitians will probably need food aid for six months, while the World Food Program and other donors so far have reached only about half a million "with reasonable quantities of food" so far.

The U.S. Navy's floating hospital, USNS Comfort, dropped anchor in view of the capital on Wednesday with about 550 medical staff, joining teams from about 30 other countries trying to treat the injured. About 250,000 people were hurt in the quake and aid groups say many people have died for lack of medical care or adequate equipment.

And the Pentagon announced that 2,000 more U.S. marines would be sent to Haiti, adding 11,500 U.S. military personnel already on the ground or on ships offshore - a number expected to reach 16,000 by week's end.

Defence Minister Peter MacKay said more than 1,000 Canadian troops were on the ground. Upwards of 2,000 were expected by next week.

"This is a tragedy of unspeakable proportions," MacKay said. "Canada and the Canadian Forces want to be there, want to assist in whatever ways possible as part of this international response.

The U.S. Geological Survey said the aftershock was centred about 60 kilometres west-southwest of Port-au-Prince and 10 kilometres below the surface.

It was a little further from the capital than last week's magnitude-7.0 quake, which the killed an estimated 200,000 people and made as many as two million homeless, according to the European Union.

Wednesday's temblor matched the strongest of 49 aftershocks of magnitude-4.5 or greater that have followed the Jan. 12 quake and USGS geophysicist Bruce Pressgrave said nobody knows if a still-stronger aftershock is possible.

"Aftershocks sometimes die out very quickly," he said. "In other cases they can go on for weeks, or if we're really unlucky it could go on for months" as the earth adjusts to the new stresses caused by the initial quake.

The shaking ripped 20-centimetre cracks in a road west of the capital near Leogane, where U.S. marines were setting up a post to aid quake victims who are sleeping in streets, culverts and driveways, often under tree branches draped with sheets to guard against the sun.

The latest quake, combined with a light rain on Tuesday, has complicated rescue efforts.

International aid teams have saved 121 people from the rubble, an unprecedented number, according to aid organization. Dr. Jon Kim Andrus, deputy director for the Pan American Health Organization, said that "countless more have been rescued by Haitians working with no equipment at all," he said.

A 69-year-old domestic worker, Ena Zizi, said she prayed constantly during her week under the rubble.

She had been at a meeting at the residence of Haiti's Roman Catholic archbishop when the Jan. 12 quake struck, trapping her in debris. On Tuesday, a Mexican disaster team pulled her to safety.

Zizi said after the quake, she spoke back and forth with a vicar who also was trapped. But he fell silent after a few days, and she spent the rest of the time praying and waiting.

"I talked only to my boss, God," she said. "I didn't need any more humans."

Elsewhere in the capital, two women were pulled from a destroyed university building. And near midnight Tuesday, a smiling and singing 26-year-old Lozama Hotteline was carried to safety from a collapsed store in the Petionville neighbourhood by the French aid group Rescuers Without Borders.

Yet the colossal efforts to help Haiti were proving inadequate because of the scale of the disaster. Expectations exceeded what money, will and military might have been able to achieve.

Governments have pledged nearly US$1 billion in aid, and thousands of tonnes of food and medical supplies have been shipped. But much remains trapped in warehouses, or diverted to the neighbouring Dominican Republic. Port-au-Prince's nonfunctioning seaport and many impassable roads complicate efforts to get aid to the people.

Aid is still being turned back from the single-runway airport, where the U.S. military has been criticized by some of poorly prioritizing flights. The U.S. Air Force said it had raised the facility's daily capacity from 30 flights before the quake to 180.

Canada is starting to ferry troops in through the airport of Jacmel, a southern coastal community that's becoming a regional hub for the delivery of aid.

The airstrip is small at present, making it a challenge for large planes to land, but it's being upgraded. Canada is providing lighting, and the navy ship HMCS Halifax offshore has radar to guide incoming and outgoing flights.

MacKay said in Ottawa that an "airfield activation team" was getting Jacmel's airstrip back in order and two C-130 Hercules aircraft carrying relief supplies had already landed early Wednesday.

There is also an agreement to use a Jamaican airport as a transit point for Canadian military flights.

Canadian doctors have started to treat local people in Jacmel. Some of the Canadian personnel on the ground came via helicopter from the Halifax offshore.

Meanwhile, officials said 13 Canadians had been confirmed dead in Haiti by Wednesday; another 543 were missing.

U.S. Defence Secretary Robert Gates said the military will send a port-clearing ship with cranes aboard to Port-au-Prince to remove debris that is preventing many larger aid ships from docking.

The UN was sending in reinforcements as well: The Security Council voted Tuesday to add 2,000 peacekeepers to the 7,000 already in Haiti, and 1,500 more police to the 2,100-strong international force.

Perhaps as important for many Haitians was the reopening of several money exchange houses on Wednesday and Haitian banks' announcement that they will reopen in rural provinces on Thursday, then in the capital on Saturday.

That will help restore the flow of money from Haitians abroad, who send home US$1.9 billion a year.

-With files from The Canadian Press.