Emergency supplies and equipment are still needed to prepare for the fast-approaching tropical storm Tomas, which could affect up to half a million people in Haiti, the United Nations said on Thursday.

The approaching storm may be the third humanitarian crisis that Haiti faces this year, coming amid ongoing efforts to assist up to a million people left homeless by the earthquake that devastated the country in January, and a cholera outbreak that erupted last month, according to the UN Office for the Coordination of Humanitarian Affairs (OCHA).

The UN agencies and its stabilization mission in Haiti, MINUSTAH, have been working with the government of Haiti and humanitarian partners to pre-position emergency supplies and equipment, including trucks, water and sanitation materials, shelter, food and non-food items in the areas most likely to be hit by the storm.

"Even with the existing pre-positioned stocks, the potential magnitude of this disaster urgently calls for additional emergency supplies and equipment," warned Nigel Fisher, the UN Humanitarian Coordinator for Haiti.

Priority needs include reinforced emergency shelter kits for 20, 000 families, 80,000 family hygiene kits, 5,000 family water kits, 5 million sachets of oral rehydration salts, water tankers, water treatment units and field tents to serve as emergency cholera treatment centers, as well as logistics equipment and radios to support ongoing public information about cholera and hurricane alerts.

The possible path of the storm keeps changing, but the latest update suggests a high probability of severe flooding, especially in coastal areas, but also throughout the country. Haitian authorities have been encouraging residents of low-lying coastal areas and camps to seek alternative shelter, where possible, with families and friends settled in safer places, according to OCHA.

"We are concerned that severe flooding will make a difficult situation in Haiti even more difficult," Fisher said. "Cholera is spread mainly through contaminated water -- so more water poses more risks. The poor sanitary conditions in many parts of the country combined with flooding and polluted waters are very likely to accelerate the infection rate."

A massive public information campaign has been rolled out to help people prepare, with alerts being sent out to internally displaced persons camps via short message service (SMS) and radio. All available stocks of tarpaulins and ropes are being used to help people in camps secure their shelters.

"We want to focus on social and economic recovery ... but we are pulled back into emergency mode," Fisher said. "We are calling on the international community to help us prepare as best as we can."