Up to 3 million people risk starvation in Haiti following back-to-back storms that have wiped out large swathes of agriculture, an aid agency has warned.

Aid workers fear the devastation could also spark more unrest in Haiti where violent protests over spiralling food prices rocked the country earlier this year and triggered the government's fall in April.

"In the next two months the situation will become more and more acute," ActionAid country director Raphael Yves Pierre said. "The risk of food riots is very imminent."

He estimated the loss of crops meant 2 to 3 million people could face starvation in Haiti - the poorest country in the western hemisphere - where over half the population survives on less than a $1 a day.

Christian Aid's country representative Prospery Raymond said the whole of the Artibonite valley had been flooded, which is where 80 per cent of Haitian rice is grown. Many families have also lost livestock.

"Rice crops were destroyed near the point of harvesting, which can only put the price of this staple food even further out of the reach of many families," he added.

"I am very worried that we will see rioting in various parts of the country over the next few weeks as people grow frustrated with the speed of the government response."

Oxfam spokesperson Kristie Van de Wetering said colleagues in Cape Haitian in the north were reporting that food prices had already doubled.

She said flooding had also submerged a key bridge in the south, preventing trucks transporting agricultural produce from reaching the capital Port-Au-Prince. Drivers had been seen throwing food away because it was spoiling.

PRECARIOUS SITUATION

Four storms - Fay, Gustav, Hanna and Ike - have hammered Haiti since mid-August, killing hundreds of people and affecting hundreds of thousands more.

But the deluge and lack of infrastructure have made it hard to deliver relief to many of the worst affected places, including the port city of Gonaives, where people had to scramble onto roofs as the waters rose.

Aid workers said tens of thousands of people were huddled in shelters with no food, water, sleeping mats or latrines.

The only way of getting relief into Gonaives is by helicopter or boat, when the sea is calm enough.

The flooding has also destroyed bridges and submerged roads in many other parts of the country.

Aid workers warned violence could erupt among survivors unless supplies arrived fast. Oxfam's Van de Wetering described the situation on the ground as very tense and precarious.

Matt Marek, head of programmes in Haiti for the American Red Cross, said the damage to infrastructure was far wore than that caused by Tropical Storm Jeanne in 2004 when 3,000 people died in Gonaives.

"If people do not have (anything) desperation is going to cause people to possibly act violently and cause more insecurity," he added.

"We have a population that have already experienced a lot of hardship ... In the long run this is going to add to the misery of the global food crisis that people have been experiencing."

Marek expected the death toll from the recent storms to rise significantly as news came in from communities that had been cut off by the flooding.

He said the storms were particularly devastating because Haiti - a country long plagued by chronic political instability and violence - had shown signs of turning a corner.

"You get two years of progress and all of a sudden you get whacked by some natural disaster. And this is not just two steps back - it's four, five, six steps back after one step forward," he said.

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