"If basic needs, particularly access to safe drinking water, are not urgently restored to all populations by the end of this week, WHO fears that outbreaks of infectious disease could result in a similar number of fatalities as occurred due to the direct impact of the tsunami," the U.N. health agency said in a statement.
"We now estimate that as many as 150,000 people are at extreme risk, if a major disease outbreak in the affected areas occurs," said WHO Director-General Dr. Lee Jong-wook. "We are extremely concerned about the ongoing lack of access to basic needs."
The most urgent need is to make sure that the 5 million people affected by the tsunami have access to safe drinking water, Lee said.
The WHO request is part of a U.N. emergency appeal to be announced by Secretary-General Kofi Annan in Jakarta, Indonesia, on Thursday.
The health agency has already sent millions of water purification tablets, basic medical supplies and surgical equipment to southeast Asia.
Although this aid is reaching some of the affected areas, many of the affected people in Indonesia's Aceh province and on Sri Lanka's eastern coast still lack adequate access to safe drinking water.
Drinking contaminated water is now the main threat to public health in the region, and can cause outbreaks of cholera, typhoid and dysentery, WHO has said.
Malaria and dengue fever are a longer-term threat, as the flooded areas become breeding grounds for mosquitoes, said World Health Organization spokeswoman Fadela Chaib.
Lee, touring the stricken community of Banda Aceh with UNICEF chief Carol Bellamy, said the size of camps formed in the Indonesian province to house survivors are smaller than usual for humanitarian disasters, reducing the risk of disease outbreaks that could multiply the death toll.
Lee said he was encouraged that mega camps such as those formed in response to the conflict in the Darfur region of Sudan have not taken shape in the area.
"A huge camp means everything will be huge — the challenge will be huge," Lee said as he visited hospitals and makeshift clinics tending the thousands of people injured in the Dec. 26 earthquake and tsunami.
About 150 small and informal refugee camps have sprung up in the province in the week since the tsunami hit.
Life-threatening infectious diseases such as cholera, typhoid and dysentry spread easily when people are packed in together.
Overcrowded living conditions also increase the risk of respiratory diseases such as pneumonia, as well as measles, influenza and meningitis, WHO said.
No disease outbreaks have been reported so far, but there is increased incidence of diarrhea in the camps for displaced people, WHO said.
Lee also reiterated that the main public health challenges facing relief workers were providing care for the wounded, disposing of bodies, and ensuring the availability of clean drinking water and sufficient sanitary facilities such as toilets or showers to fend off disease.
Earlier, hospital staff in Banda Aceh said diarrhea, especially in children, was a growing problem since the disaster, but stressed that there had been no outbreaks of cholera, typhoid or dysentery.
Many people sustained wounds that have since become infected, and some of those injuries have turned gangrenous forcing surgeons to amputate limbs, they said. There were no exact numbers on gangrene cases.
Many people who inhaled dirty water when the tsunami washed over the region were suffering from pneumonia, they said.
Meanwhile, logistical problems continue to hamper the unprecedented global effort to deliver aid to millions of people suffering from tsunami damage, particularly in Indonesia's Aceh province because of the severe damage to infrastructure.
"The challenge for the moment is logistics," said Simon Pluess, spokesman for the World Food Program, adding that it is particularly difficult to access the western coast of Sumatra, which was closest to the epicenter of the earthquake.
The United Nations Tuesday started a helicopter survey of the "devastated" coastline, where soft earth has previously prevented aircraft landings, said Ron Redmond, spokesman for the U.N. High Commissioner for Refugees.
"We have huge logistical challenges in Aceh, where we are starting operations from scratch," Redmond said. "We're looking at the possibility of using both helicopters and barges."
Although UNHCR does not normally respond to natural disasters, it is helping other agencies because of the massive scale of the crisis, Redmond explained.
The refugee agency is airlifting hundreds of tons of supplies from its stockpiles in Copenhagen, Denmark, and the first shipments are now being transported from Indonesia's capital Jakarta to Aceh, Redmond said.
The international Red Cross said Wednesday it needs at least $556 million for long-term aid to tsunami victims.
"Our initial appeal has already been covered but the needs are much greater," said Susan Johnson, director of the International Federation of Red Cross and Red Crescent Societies.
The Red Cross anticipates spending the money over a period of more than five years, Johnson said.
"The scope of this disaster calls on the world's humanitarian agencies to act quickly, effectively and with a long term perspective," Johnson added.