Reports say that aid agencies have begun what the United Nations said would become the biggest relief effort the world has ever seen, following Sunday's earthquake and tsunamis that struck coastal south Asia.

The UN called on countries to rush aid to victims of the tsunamis that hit ten countries causing over 24,000 deaths.

• How you can help the victims

UN Under Secretary Jan Egeland said the natural disaster could be the costliest in history and "many billions of dollars" might have to be spent.

"Hundreds of thousands have lost everything, and millions face a hazardous future because of polluted drinking water, lack of sanitation and no health services," he said.

The International Red Cross, which reported 23,700 deaths in Asia, said it was concerned that diseases like malaria and cholera could add to the toll.

More than one million have been displaced, the Red Cross said.

UN agencies have also begun relief operations in countries hit by the killer quake.

Also Read Nagapattinam resembles mass grave 25,000 could be Indonesia toll alone Toll crosses 24,000 Contribute to PM's fund Where were you when disaster struck?

Though India has not sought international help and plans to meet the emergency needs through is own capacities, United Nations Development Fund and United Nations Children Fund have begun providing assistance to several other countries and plan to step up aid over next few days. Sri Lanka and Indonesia are likely to have greatest need for humanitarian assistance, said UN reports.

UNICEF said hundreds of children were separated from parents in the chaos following sudden flooding and it is trying to establish the extent of the problem and measures needed.

In India, UNICEF said it is supporting relief efforts led by the state and local authorities as well as the central government.

In Tamil Nadu, UNICEF is providing hundreds of thousands of water purification tablets, 1600 community water tanks (500 litres each), 200,000 sachets of oral rehydration salts, medical supplies sufficient to serve 30 health centers, and 30,000 blankets.

Meanwhile, a top UN official said small states were "most vulnerable" to disasters like the tsunami tragedy. He urged the international community to pay special attention to their needs and help them rebuild their economies.

The Under Secretary General of the UN International Meeting on Small Island Developing States, Anwarul K Chowdhury, also called for giving special consideration to issues of climate change, saying the tragedy comes on the heels of several "climatic disasters".