Relief workers are racing against time in South Asia, where United Nations officials warn the death toll from last week's tsunami will rise unless survivors get access to clean water and other supplies.

The international aid effort was hampered today (Tuesday) when the main airport at Indonesia's Sumatra island closed for several hours after a relief plane hit a water buffalo on the lone runway. The airport has reopened, but a U.S. military relief officer said the accident could not have happened at a worse time.

The World Health Organization said today some five million people in the region face the threat of cholera and other diseases without clean drinking water.

The U.N. agency estimates that more than 500,000 people were injured in the catastrophe and need medical treatment.

The overall death toll in the region stands at 145,000, with more than 94,000 of the fatalities in Indonesia. U.N. Relief Coordinator Jan Egeland warns that many villages on Sumatra's low-lying west coast "are gone," sparking fears that the death toll on the Indonesian island will rise significantly.