Britain will give up to £5 million of initial aid to help those affected by Hurricane Matthew in Haiti.
Almost 300 people have died and more than 350,000 have been affected after 145mph winds savaged the Caribbean country.
Temporary shelters, water purification kits and solar lights from the UK will arrive in Haiti in the next few days to help 12,500 of the most vulnerable people.
Experts from the Department for International Development (DfID) are already in the region helping UN and NGO partners address the country's most pressing needs.
International development secretary Priti Patel said: "Tragically, with the death toll continuing to rise we are now beginning to see the full scale of the devastation wrought by the worst storm to hit this region in almost a decade.
"The absolute priority right now is to reach those who are injured and provide them with water, sanitation, shelter and protection."
Helene Robin, head of Handicap International's emergency response, said there was a "heightened risk of epidemics such as cholera" following the torrential rain and violent winds, and warned that stocks of medication and prevention equipment were probably destroyed by the hurricane.
She said: "Hundreds of thousands of people are still cut off from the world and all humanitarian aid. They need immediate assistance because they probably lack drinking water and do not have roofs to protect them from the weather."
Unicef said it was unclear how many of an estimated 500,000 children who live in the worst affected areas hit by Hurricane Matthew in Haiti were in need of urgent assistance.
Marc Vincent, Unicef representative in Haiti, said: "We're still far from having a full picture of the extent of the damage. We are hoping for the best, but bracing for the worst.
"The more information we can gather on the ground, the better we can support the Government's efforts to help children and their families cope with the scale of destruction."