"Many people have suffered multiple fractures and internal injuries. By any stretch of the imagination it is going to be incredibly difficult," Jon Andrus of the Pan American Health Organisation, the regional arm of the World Health Organisation, told Reuters news agency.
"The population in Haiti was already vulnerable and faced enormous health threats."
He said the greatest threats to public health resulted from the scores of people with wounds and injuries caused by collapsing buildings during the quake.
The other threat comes from diseases of insanitation among the makeshift camps that have sprung up for the hundreds of thousands left homeless.
Al Jazeera's Teresa Bo reports from a slum district hit hard by last week's earthquake
"Tetanus, gangrene are immediate threats, as is the spread of measles, germs causing meningitis and other infections with crowding."
Corpses were being burned or dumped in mass graves but Andrus told a news conference on Monday that this was unnecessary and could damage the mental state of survivors.
Doctors working in the capital Port-au-Prince also said that many children, the weak and elderly are at risk of dying unnecessarily from diarrheal disease that could be easily treated with water and rehydration salts.
But logistical problems - including a severe bottleneck at the capital's small airport, the main entry point for aid - have meant that aid supplies are just starting to reach those most in need.
Jacques Lorblanches, a surgeon sent by the humanitarian group Doctors of the World, told AFP news agency that he had lost count of the number of amputations performed in the past 48 hours in Haiti.
"I have never seen anything like this - infected wounds full of larvae," he said.
"I did my first amputation with three forceps, five scissors and a scalpel, without water, and just a flashlight to illuminate the injury."
From Saturday to Monday, out of 30 operations performed by the French team at the General Hospital in Port au Prince, 28 ended in amputation.
The story was the same elsewhere in the city.
"Of the 48 operations performed over two days, almost all were amputations," said doctor Amit Assa, at another field hospital set up by an Israeli team on the outskirts of the capital.
As victims arrived with gangrenous limbs crushed days ago by concrete walls or metal beams, doctors said it was inevitable that many surgeries would end in amputation.
Meanwhile, many of the wounded have waited for days for treatment, often without even simple painkillers which are also in desperately short supply.
"There is gangrene everywhere and you amputate on the go," said Hans Van Dillen, the head of a Doctors Without Borders (MSF) mission in Haiti.
The group said its staff had been conducting 100 surgeries per day under difficult conditions.
"On day six, you conduct radical surgery. There's nothing more you can do," Van Dillen said.
And with the number of casualties needing immediate treatment increasing rapidly - skull fractures, severe burns, open wounds, "we are completely overwhelmed," he said.