The charity's Sri Lanka country director Chris McIvo said: "We're particularly anxious we could start seeing a further increase in the number of dengue cases because of the stagnant water that the floods will leave in their wake".
The fist Indian navy ship with rescue and relief materials arrived this morning at the Port of Colombo.
Indian medical teams and rescue personnel also arrived in the capital of Colombo.
"Many thousands are displaced and are trying to come to terms with what has happened with this huge deluge of water", Al Jazeera's Minelle Fernandez, reporting from the southern town of Agalawatte, said.
The Sri Lanka Air force has also deployed one helicopter for Neluwa and have arranged 3 other for the rescue operations. According to residents, many people remained locked in their villages as a result of natural disasters.More news: Taiwan ranks 41st in 2017 Global Peace Index
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Sri Lanka stepped up its military-led relief operations Saturday as almost half a million people were displaced after a monsoon deluge killed at least 113 people. At least 100 deaths have been confirmed by the Lankan government while about 99 are still missing.
Floods and landslides fueled by torrential rains have so far left 91 people dead and another 110 missing in Sri Lanka. The second will reach on Sunday.
The Sri Lankan government has appealed to the United Nations and neighbouring countries for assistance.
Chief of Meteorological Department R S Jayasekera said while the peak of the monsoon had passed, more rains are expected during the next few days.
In 2003, the powerful Southwest monsoon wrecked 10,000 homes and killed at least 250 people.
Sri Lanka, which has been heavily deforested for cash crops, often witnesses landslides during the monsoon season.