The scale of a cholera outbreak in Haiti after Hurricane Matthew may be underreported because remote areas are cut off, a United Nations official in charge of controlling the disease said on Tuesday, adding protests over slow aid made the problem worse.

David Nabarro, a special advisor to the U.N. Secretary-General who was previously in charge of the global body's response to the water-borne disease, said he was concerned sick people were not being treated.

He called on donor nations to fund the U.N. response to the outbreak, a sensitive topic in Haiti because the disease was accidentally introduced to the Caribbean country by U.N. peacekeepers and has since killed more than 9,000 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 desperate 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 journalists.

"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, U.N. 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 7.0-magnitude 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 U.N. had received just $15 million of $120 million it asked for in an appeal after the hurricane, further straining relief efforts in Haiti.

He did not say how much of that funding had been allocated to the cholera response, but said he would encourage donors to step up funding when he returned to New York later on Tuesday.

"It is difficult to have a good U.N. response... if we don't have enough money," Nabarro said.

The U.N. is soon launching a new plan intended to improve cholera response and water and sanitation infrastructure in Haiti, and to provide material assistance to victims.

The World Health Organization said last week it was sending 1 million cholera vaccines to Haiti.