The United Nations warned on Monday of epidemics within days unless health systems in southern Asia can cope after more than 15,500 people were killed and hundreds of thousands left homeless by a giant tsunami.

Aid agencies round the world rushed staff, equipment and money to southern Asia after huge waves, triggered by a massive underwater earthquake, pummeled and swamped coastal communities in at least six countries Sunday.

“This may be the worst natural disaster in recent history because it is affecting so many heavily populated coastal areas ... so many vulnerable communities,” the U.N.‘s Emergency Relief Coordinator Jan Egeland told CNN.

“The longer term effects may be as devastating as ... the tsunami itself,” said Egeland.

“Many more people are now affected by polluted drinking water. We could have epidemics within a few days unless we get health systems up and running.

“Many people will have (had) their livelihoods, their whole future destroyed in a few seconds.”

Sri Lanka, India and Indonesia suffered the highest death tolls but Thailand, Malaysia, Myanmar and Bangladesh were also hit by the surging walls of water. Government officials estimate in Sri Lanka alone, 800,000 people were forced from their homes.

Experts said the top five issues to be addressed were water, sanitation, food, shelter and health.

“ROTTING BODIES”

“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 Christian Aid emergency officer Dominic Nutt.

Some affected areas have had communications cut. Others are so remote it is impossible to know the extent of the damage.

“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,” said Phil Esmond, head of Oxfam in Sri Lanka.

The Geneva-based International Federation of Red Cross and Red Crescent Societies said it was seeking an immediate $6.5 million for emergency aid funding.

“This is a preliminary appeal. It will be revised after exact needs are evaluated,” said Simon Missiri, head of the federation’s Asia Pacific department.

Earlier, the federation released $870,000 from its disaster relief emergency fund to get assistance moving to the region.

“The biggest health challenges we face is the spread of waterborne diseases, particularly malaria and diarrhea, as well as respiratory tract infections,” said the Red Cross Federation’s senior health officer Hakan Sandbladh.

The federation said it would send an assessment and coordination team to Sri Lanka, and had on standby several emergency response units specialized in water and sanitation as well as field hospitals.

The United States said it would offer “all appropriate assistance” to Asian countries, with some aid already on its way to Sri Lanka and the Maldives.

“We’re prepared to be very responsive,” said State Department spokesman Noel Clay.

The European Union pledged an initial three million euros ($4 million) and local news agency Belga said Belgium had allocated its own 500,000 euros in emergency aid to be distributed by Red Cross bodies and the EU.

Britain said it had offered what it called practical help.

“What we don’t know is the number of people who’ve been displaced, and what infrastructure has been affected. That’s the critical point,” said Titon Mitra, emergency response director for the CARE aid agency in Geneva.