* 'Uncommonly fierce and prolonged' storm- Guterres
* Mozambique's affected area the size of Luxembourg
* Aid workers prepare for disease
* WHO sends 900,000 doses of cholera vaccine (Gives range of IMF emergency loan)
By Emma Rumney and Stephen Eisenhammer
BEIRA, Mozambique, March 26 (Reuters) - Cyclone Idai's deadly hit has left some 1.85 million people in need of assistance in Mozambique in a catastrophe that United Nations chief Antonio Guterres said on Tuesday rang "yet another alarm bell" about climate change.
Guterres described Idai, which flattened homes and caused massive flooding after slamming into Mozambique near the port of Beira on March 14, as "an uncommonly fierce and prolonged storm".
The cyclone ripped through neighbouring Zimbabwe and Malawi, killing at least 686 people across the three southern African countries. In hardest-hit Mozambique, tens of thousands of homes were destroyed and hundreds of thousands of people were displaced across an area of some 3,000 square km (1,200 square miles) - roughly the size of Luxembourg.
"At least one million children need urgent assistance and this number may well grow. We fear that whole villages have been washed away in places we have yet to reach," Secretary-General Guterres told reporters at the United Nations.
Some reports said $1 billion worth of infrastructure had been destroyed, he said.
The storm was, Guterres said, "yet another alarm bell about the dangers of climate change".
While scientists say single weather events cannot be attributed to climate change, they say global warming is causing more extreme rainfall and storms, sweltering heatwaves, shrinking harvests and worsening water shortages around the world.
Receding flood waters in Mozambique have allowed greater access, and a greater sense of how much people have lost. Thousands of people, stranded for more than a week by the flooding, are now being moved to safer shelters.
"We're now going out on the ground, dropping people off from helicopters to determine what the critical needs are," said Sebastian Rhodes Stampa, coordinator in the U.N. Office for the Coordination of Humanitarian Affairs.
BIG, DENSE POPULATION
The relief focus has increasingly turned to preventing or containing what many believe will be inevitable outbreaks of malaria and cholera. No cholera cases have yet been confirmed, but health workers have reported an upsurge in cases of diarrhoea - a symptom of the disease.
"We are testing as we go," said Rob Holden, the World Health Organization's (WHO) incident manager in the capital Maputo. "But nonetheless we are treating acute watery diarrhoea, it's the same as treating cholera. That's just the diagnosis."
Dozens of people queued in front of a clinic in Beira's Munhava district on Tuesday, as nurses wearing surgical masks handed out a chlorine solution to prevent the spread of diseases.
"There is a big population, dense population in Beira," said Gert Verdonck, emergency coordinator for Medecins Sans Frontieres (MSF). "Of course any spread of any kind of epidemic will be a lot quicker here."
The WHO is dispatching 900,000 doses of oral cholera vaccine from a global stockpile. The shipment is expected to arrive within 10 days, and a first round of vaccinations will target 100,000 people.
Cholera is spread by faeces in sewage-contaminated water or food, and outbreaks can develop quickly in a humanitarian crisis where sanitation systems are disrupted. It can kill within hours if left untreated.
The United Nations World Food Program (WFP) has designated Mozambique a level three emergency, placing it on a par with Syria, Yemen and South Sudan. The agency is preparing to feed 1.7 million people in Mozambique, which has a population of around 30 million.
The United Nations is appealing for $282 million to fund the first three months of the disaster response in Mozambique, and a total of $337 million. So far, only 2 percent of that amount has been funded.
The International Monetary Fund (IMF) said on Tuesday it will consider emergency financial assistance in Mozambique under an IMF Rapid Credit Facility. While it was still early to precisely assess the cyclone's macroeconomic effects and reconstruction costs, "these will be very significant", the IMF said in a statement.
IMF chief of the mission visiting Mozambique, Ricardo Veloso, said it would loan Mozambique $60 million to $120 million under its loan facility to help in reconstruction efforts.
Mozambique admitted in 2016 to $1.4 billion of previously undisclosed loans, prompting the IMF to cut off support and triggering a currency collapse and debt default.
SEARCHING THROUGH RUBBLE
In Zimbabwe, where 179 people have died, another 329 people were still unaccounted for on Monday.
In Chimanimani district, villagers used hoes and shovels to dig through debris on Tuesday and search for missing relatives believed buried in mudslides unleashed by the cyclone.
One family has spent a week digging day and night for four relatives, in what was once a settlement of 500 people but has been reduced to rubble.
Large rocks, some more than two metres (six feet) high, rolled down from a nearby mountain at high speed and are all that remains after the storm swept away a police camp, houses and an open market.
"I am an orphan now and I am so much in pain because I lost my brother who looked after me. He was more of a father to me," said Sarah Sithole, 32, whose policeman brother was washed away while on night duty at the police station.
"We will continue searching until we find him and bury him. We will not rest," she said, her hands and feet covered with red soil.
Around 95 percent of roads in affected districts have been damaged, impeding access to rescuers with earth moving equipment. Zimbabwe has asked South Africa to send search dogs, a local government official said.
The WFP said it will aim to distribute food assistance to 732,000 people in Malawi and 270,000 in Zimbabwe.
(Additional reporting by Gift Sukhala in Chimanimani, Zimbabwe, MacDonald Dzirutwe in Harare, Stephanie Ulmer-Nebehay in Geneva, Michelle Nichols at the United Nations, Manuel Mucari in Maputo and Tanisha Heiberg in Johannesburg Writing by Joe Bavier Editing by William Maclean and Frances Kerry)
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