For its part, the United Nations Environmental Programme (UNEP) said the most urgent issues include waste management, mass burials and disposal of demolition material.

Experts say medical clinics have 12-day patient backlogs, untreated injuries are festering and makeshift camps housing thousands of survivors could foster disease.

"The next health risk could include outbreaks of diarrhoea, respiratory tract infections and other diseases ...," said Dr Greg Elder of Doctors Without Borders in Haiti.

'Sisters vomited'

Overcrowding in parks and other areas where people set up temporary shelters has stressed sanitation and hygiene beyond the breaking point.

"People go to the toilet everywhere here and I'm scared of getting sick," 12-year-old Judeline Pierre-Rose told Reuters.

	The need for medical aid is great

"My twin sisters vomited last night," she added.

"It's dangerous because health is a very precious thing and you can't have all these people living near trash and dead bodies," said another resident, Gelin Wesnel, 34.

Apart from shelter and food, the immediate focus is on medical assistance and clear water and sanitation for the nearly 3 million people affected.

Mobile clinics

A spokesman for the Red Cross in Haiti told the BBC that thousands of families were now receiving fresh water supplies and that makeshift hospitals were making good progress.

"We are delivering half a million litres of fresh water every day now," Paul Conneally said.

"For the field hospitals, they have capacity for 300 each a day. "We have mobile clinics out there. Also, three of them treating 600 people a day: very important, particularly when we talk about disease prevention and reduction."

Dr Elder said that patients were dying of sepsis from untreated wounds and that some of the group's posts had 10- to 12-day backups of patients.

UN humanitarian chief John Holmes said agencies were "straining every nerve" to get the right help in place.