Torrential rains in Sri Lanka have left more than 17,000 people displaced and 24 dead in what is being considered the worst natural disaster in the country since the 2004 tsunami.

Agencies have estimated that over half a million people have been affected by the rains, which have been ongoing for nearly a week now.

The hardest hit region was the western district of Gampaha, where some 17,000 people were evacuated. A state of emergency was also declared in the Nuwara Eliya district in the hill country, where officials are concerned about the possibility of landslides.

Abdul Hameed Mohamed Fowzie told Integrated Regional Information Networks (IRIN) that the amount of rain came as a "shock not only to the government but to everyone."

"We in government are trying to provide relief for people despite their locations - at IDP camps or at affected residences," Fowzie said

The United Nations have joined the government in the relief effort, pledging to provide food for 400,000 victims of the floods.

Other groups providing direct assistance are Caritas and World Vision, who are also focusing on providing emergency food assistance to those displaced.

"Dry rations distributed by the government with the assistance of WFP had reached only 5,000 people, but there are over 50,000 people who have been displaced in the Wattala area alone and the need for food is critical at this point," Ajith Gomis, manager of World Vision's Negombo Area Development Programme, said.