THAINGKHALI, Bangladesh -- UN agencies and Bangladesh's health ministry began a massive cholera immunization campaign Tuesday to stem a possible outbreak of the water-borne disease among more than a half million Rohingya Muslims who have fled violence in Myanmar to neighbouring Bangladesh.

No cases of the potentially life-threatening diarrheal disease have yet appeared in makeshift camps in Bangladesh.

UNICEF said the vaccination campaign involves 900,000 doses of the vaccine delivered by more than 200 mobile teams, making it the second-largest oral vaccination campaign of its kind, after another in Haiti last year. The campaign will also target residents of older refugee camps that have existed in Bangladesh since the 1990s.

The World Health Organization has tallied some 10,000 cases of diarrhea among the refugees, and says unhygienic and cramped conditions raise the potential for a cholera outbreak.

"Nobody wants another emergency within this existing emergency that we are facing with the Rohingya refugees here. Wherever there are refugee crises, there is always this crisis of cholera outbreak and so on, because people live in a very crowded position, there is scarcity of services like water, sanitation, and everything," said Sakil Faizullah, a spokesman for UNICEF.

Waves of Rohingya Muslims have crossed over to Bangladesh since Aug. 25, when security forces in Buddhist-majority Myanmar responded to militant attacks with a broad crackdown that the UN has described as "textbook ethnic cleansing."

The UN refugee agency says Bangladeshi border guards reported over 11,000 Rohingya crossed the border on Monday.

In the southern parts of Bangladesh they have joined hundreds of thousands of Rohingya who were already living in miserable conditions in tightly packed camps. Now with hundreds of thousands more people, the pressure on what little resources the old camps had is incredible.

Entire new towns of shanties have sprung up across the area. Clean water and toilets are a rarity and there are fears that diarrheal disease will spread.

"We get a very bad smell from the bathrooms here. What can we do though? This is our fate," said Gulbahar, a Rohingya refugee standing in line to get vaccinated who uses one name.

The first round of the cholera vaccination campaign will cover 650,000 people age one and older. The second round will begin Oct. 31 and target 250,000 children between one and five years with an additional dose of the vaccine for added protection.

But aid workers are circumspect about how much they can do to protect from epidemics in camps that are bursting at the seams.

"No government, no aid agency in the world can say that they are fully prepared to cater for these many numbers of people," Faizullah said. "And as we are seeing, more and more Rohingya refugees are crossing the border and coming here."

Associated Press writer Jamey Keaten in Geneva contributed to this report.