The public are being asked to help clean drinking wells contaminated by the monsoon floods, as doctors work to prevent diseases such as cholera from spreading.
Widespread flooding and devastating mudslides in Sri Lanka have killed at least 202 people and displaced more than half a million, the government has said.
The state-run Disaster Management Center said around 600,000 people remained temporarily homeless on Wednesday following last Friday’s landslides and floods.
Al Jazeera’s Minelle Fernandez, reporting from Matara, around 160km south of the capital Colombo, said aid agencies were struggling to distribute fresh food to rural areas.
“In the capital, shops and supermarkets are running out of supplies as people are coming in and hoovering up items,” she said.
“While waters are receding in some areas, there are still some parts that are 10 to 12 feet under water.”
Foreign Minister Ravi Karunanayake said 16 countries had sent medicines and relief supplies to assist those driven from their homes.
“We also have a lot of enquiries from other countries and organisations wanting to know our immediate needs. We are moved by the spontaneous response,” Karunanayake told reporters.
The UN warned that with an increasing number of displaced people and a lack of space in temporary shelters, many people were at risk of disease.
Sri Lanka has seen a significant increase in mosquito-borne dengue fever this year, with more than 125 deaths.
Al Jazeera’s Charles Stratford, reporting from Kalutara city, said residents were still without access to water and electricity and heavily reliant on voluntary services.
“The UN has pledged more money, neighbouring countries have also come forward. The Indian navy has supplied three ships full of aid and China and Japan have also put money forward,” he said.
“The Sri Lankan military is also saying it’s doing all it can. Search-and-rescue operations are still ongoing. But residents are saying if it wasn’t for private organisations and people coming forward, the government and military would be finding it even harder to deal with this crisis.”
Mudslides have become common during Sri Lanka’s summer monsoon season as forests across the tropical nation have been cleared for export crops such as tea and rubber.
A massive landslide a year ago killed more than 100 people in central Sri Lanka.