PORT-AU-PRINCE, Haiti (AP) - A powerful aftershock sent Haitians screaming into the streets, 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 January 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 Co-ordination 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.

Hundreds of thousands of Haitians remain homeless, hungry and in mourning - most still waiting for the benefits of a nearly $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 said 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 US Navy’s floating hospital, USNS Comfort, dropped anchor in view of the capital 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 US Marines would be sent to Haiti, adding 11,500 US military personnel already on the ground or on ships offshore - a number expected to reach 16,000 by week’s end.

At a golf course where US troops have been trying to help 25,000 people living under sheets of plastic and old cloth, soldiers and quake victims alike raced for open ground as the quake began.

A slow vibration intensified into side-to-side shaking that lasted about eight seconds.

Some in Port-au-Prince said the far stronger January 12 earthquake seemed to last for 30 seconds.