The next few days are critical to stop the spread of diseases killing even more people following the Indian Ocean tsunami.
Aid could struggle to reach people affected by the tsunamis
Cholera, malaria, dengue fever and diarrhoeal diseases could take hold in the next week unless fresh drinking water and medical supplies reach the areas affected by the disaster.
But relief agencies have warned there may be problems accessing areas, and ensuring people have access to clean water.
Unicef executive director Carol Bellamy said "time is of the essence right now".
"Hundreds of thousands of people fought to survive the tsunamis. Now we need to help them survive the aftermath.
"For children, the next few days will be the most critical."
Dr David Nabarro, of the World Health Organization (WHO), agreed.
He warned if relief did not reach people by the end of the week there would be a rise in diarrhoeal diseases within two weeks.
Western governments have promised more than £50m of relief while aid agencies are also sending supplies of medicine and water purification equipment.
The WHO has already warned the numbers killed by subsequent diseases could exceed the 60,000 estimated to have died when the tsunamis hit on Sunday.
Corrine Woods, who is helping co-ordinate Unicef's aid response in India, said access to areas affected by the tsunamis was a concern.
She said 2,500 water storage tanks, each holding 500 litres, water purification powder and rehydration tablets were on their way to affected states.
Speaking from India, she also told BBC News: "We are also facing a challenge to make sure people know what precautions they should take.
"Water supplies have been contaminated and we are sending people around the areas affected to tell people how they can get hold of fresh drinking water once the supplies arrive."
Experts say getting clean water, and managing the disposal of human waste are the key priorities.
But they say the accumulation of dead bodies is not as great a risk as people believe, and it is the living who pollute the water.
In Sri Lanka, Tamil Tiger rebels have complained aid is not reaching areas under their control.
And Ted Chaiban, speaking from Unicef's office in the capital, Colombo, said Sri Lankans and the relief effort was at risk from landmines.
"Mines were floated by the floods and washed out of known mine fields, so now we don't know where they are and the warning signs on mined areas have been swept away or destroyed."
A Medicins Sans Frontieres spokesman said: "Humanitarian efforts to reach populations are always frustrated during such natural emergencies.
"Communications and transport are often severely interrupted.
"Telephone lines have been destroyed and many airports are either damaged or flooded."
The Associated Press has reported that aid is piling up at distribution points into the Aceh province in Indonesia because of impassable roads.
Corruption also remains a concern.
David Macdonald, Oxfam's Indonesia programme manager, said supplies could be taken by private hospitals, the military or corrupt traders who would then sell it.
A spokesman for the International Federation of Red Cross and Red Crescent Societies said aid agencies were also facing a challenge dealing with 11 disaster zones with "different aid needs".
"There is no point just throwing aid as quickly as possible at the countries affected.
"We need to get logistics teams on the ground, which is happening, to find out what the local situation is and how help can be managed."