International rescue missions and aid began arriving in the disaster-hit regions of south Asia yesterday.
As well as providing medical supplies and shelter, one of the main concerns is to avoid an outbreak of disease by providing clean water.
"We've had reports already from the south of India of bodies rotting where they have fallen and that will immediately affect the water supply especially for the most impoverished people," said Dominic Nutt emergency officer for Christian Aid.
Specialists from Britain and other European Union countries flew to Sri Lanka and the Maldives to assess what emergency aid was needed. The British Government is also considering chartering helicopters for search-and-rescue missions.
The International Federation of the Red Cross launched an appeal for donations, and said it was seeking an initial £3.4m, after releasing £455,000 from its disaster relief fund to get assistance moving in the region.
The EU made an immediate donation of ¤3m (£2.1m), the United States confirmed it would also be supplying aid and Britain's Department for International Development offered £52,000 to the World Health Organisation's response team.
Much of the focus for aid was on Sri Lanka; both the worst hit country and one blighted by a chaotic and under-developed emergency services system.
Oxfam has already begun distributing water and food to the worst hit areas. Head of the charity in Sri Lanka Phil Esmond said: "This is a massive humanitarian disaster and the communications are so bad we still don't know the full scale of it. Unless we get aid quickly to the people many more could die."
Save The Children said it had a team flying out last night from Britain to help workers already in Sri Lanka, and the International Rescue Corps said it had a team on standby.
David Alexander, international director of the British Red Cross, warned: "We are in for a big emergency disaster response, and we will be at it for many months to come.
"In this situation we have the unusual aspect that we have got wealthy international tourists who are affected," he added. The Red Cross Federation said it would be sending medical supplies for 100,000 people in Sri Lanka.
"The biggest health challenge we face is the spread of waterborne diseases, particularly malaria and diarrhoea, as well as respiratory tract infections," said Hakan Sandbladh, the federation's senior health officer in Geneva.