The aftermath of Hurricane Matthew is hampering the response to Haiti's cholera outbreak, with experts fearing the disease's extent may be worse than feared.
The scale of Haiti's cholera outbreak after Hurricane Matthew might be under-reported because remote areas are cut off, and protests over slow aid have made the problem worse, the UN says.
A special adviser to the United Nations secretary-general, David Nabarro, said he was concerned sick people were not being treated.
He called on donor nations to fund the UN response to the outbreak, a sensitive topic in Haiti because the disease was accidentally introduced to the Caribbean country by UN peacekeepers and has since killed more than 9000 people.
Some roads in southwestern Haiti remain impassable after this month's storm and rising anger about the slow pace and uneven distribution of aid have led people to barricade roads and, at times, loot humanitarian convoys.
"We don't know if there are many people with the problem of cholera in the areas that we cannot access and that is why I ask the people, let us access everywhere," Nabarro told reporters.
"We fear that there are people in caves, in other places, without help and they are perhaps sick."
A teenager was shot in the chest on Tuesday when Haitian police used firearms against a group trying to loot a truck in Les Cayes, a centre for aid distribution in the southwest, regional police chief Luc Pierre said.
The incident further spurred anger against the apparent use of live ammunition by police.
At the weekend, UN Secretary-General Ban Ki-moon witnessed the looting of a food aid container during a brief visit to Les Cayes.
The office of interim President Jocelerme Privert on Tuesday alleged supplies where being appropriated to be handed out in a "political or partisan manner".
Haiti's cholera epidemic began in 2010, when Nepalese peacekeepers poured infected sewage in a river shortly after a devastating earthquake.
The disease has now flared in some hurricane-affected areas, mostly in the southwest of the country, as floods contaminated drinking water after the Category 4 storm.
Nabarro said the UN had received just $US15 million ($A20 million) of $US120 million it asked for in an appeal after the hurricane, further straining relief efforts in Haiti.