Ireland has pledged to provide humanitarian aid
Nearly 900 people have been killed after Hurricane Matthew battered Haiti, with outbreaks of cholera claiming more lives in the aftermath.
Christian Aid said the situation was "critical" - and was likely to remain that way for 18 months.
Prospery Raymond from the charity said: "The south part of Haiti is really affected by Hurricane Matthew.
"The population are really in need of water, shelter, materials. Compared to the earthquake this emergency is very, very critical."
The number of people reported killed has continued to rise as information has trickled in from remote areas previously cut off by the storm.
The latest total stands at 877, according to local officials.
Rural clinics were inundated with patients suffering injuries including broken bones.
Seven people have died of cholera in the town of Anse-d'Hainault on the western edge - possibly because of sewage becoming mixed with flood water.
A further 17 cases of the disease were reported in Chardonnieres on the south coast.
The Pan American Health Organization said it expected cases of cholera to "surge" because of the effect of "massive flooding" on "sanitation infrastructure".
Roads were flooded by sea and river water and cellphone networks got cut off as winds reached speeds of 145 mph.
Thousands of homes have been destroyed, while more than 60,000 people have entered shelters.
The Foreign Affairs Minister Charlie Flanagan has announced Ireland will provide humanitarian aid to Haiti in response to the hurricane.
Minister Flanagan said: "The damage and destruction caused by Hurricane Matthew in Haiti in recent days is another devastating blow to communities previously affected by the earthquake in 2010.
"Initial rapid assessments indicate that over 350,000 people are in need of humanitarian assistance and some 20,000 people have been displaced from their homes.
"There are increasing concerns that flooding will exacerbate pre-existing epidemics such as cholera, dengue fever and Zika.
"The death tolls continues to rise and we know that many thousands of people have lost their homes and possessions in the aftermath of the strongest storm to hit the region in a decade."
The US military is sending helicopters to Haiti along with the USS Mesa Verde.
The amphibious transport ship is carrying food, medicines, baby formula, nappies and first aid supplies.
It can also produce 72,000 gallons of fresh portable water each day. There is a surgical team on board and two operating theatres.
Across the country, the International Red Cross said tens of thousands of people needed help as it launched an appeal.
UNICEF Ireland has also launched an emergency appeal for the children of Haiti after assessments indicated they need US$5m to carry out essential life-saving work.
In the farming village of Chantal, the mayor said 86 people had died. Most had perished after houses were crushed by trees.
The mayor said another 20 people were missing.
"Devastation is everywhere," said Pilus Enor, mayor of the town of Camp Perrin on the peninsula's south shore.
"Every house has lost its roof. All the plantations have been destroyed," he said.
"This is the first time we see something like this."
The southern city of Jeremie is said to have suffered "complete destruction", while a bridge over the La Digue river in Petit Goave has been destroyed.
In Les Cayes - home to a major port - a cathedral roof has been blown off, and banana and mango crops been destroyed.