The worst-ever outbreak of dengue fever has killed nearly 300 people in Sri Lanka, with aid agencies warning against further spread of the mosquito-borne viral disease.

The Sri Lanka Red Cross Society and the International Federation of Red Cross and Red Crescent Societies said on Monday that they were scaling up emergency assistance to the island nation, as the number of dengue cases topped 100,000.

"Dengue patients are streaming into overcrowded hospitals that are stretched beyond capacity and struggling to cope, particularly in the country’s hardest hit western province," Red Cross/Red Crescent said in a statement.

The Red Cross announced it was increasing its budget by $300,000 to cope with the unprecedented outbreak.

The ministry of health said the number of dengue infections has climbed above 100,000 since the start of 2017, with 296 deaths.

That figure is nearly double the previous year, with about half those cases reported from the island's west, including the capital Colombo.

Health officials blamed the current crisis on recent monsoon rains and floods that have left pools of stagnant water and rotting rain-soaked rubbish, ideal breeding sites for mosquitoes that carry the virus - which causes flu-like symptoms and can develop into the deadly hemorrhagic dengue fever.

The Red Cross warned that ongoing downpours and worsening sanitation conditions raise concerns the disease will continue to spread.

According to the World Health Organization (WHO), dengue is one of the world's fastest growing diseases, endemic in 100 countries, with as many as 390 million infections annually.

Early detection and treatment save lives when infections are severe, particularly for young children.

"Dengue is endemic here, but one reason for the dramatic rise in cases is that the virus currently spreading has evolved and people lack the immunity to fight off the new strain," Novil Wijesekara, head of health at the Sri Lanka Red Cross said in a statement.

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The unprecedented outbreak of the disease has forced the closure of schools as tens of thousands of volunteers have joined soldiers in identifying mosquito breeding grounds.

"Instructions have gone out to all schools today to coordinate with local authorities and look for any mosquito breeding areas in their immediate neighbourhood," an education ministry official said.

Troops have already been spreading out across the island to help local councils drain pools of stagnant water to ensure mosquito larvae have nowhere to flourish.

Last week, Australia said it was donating $1.2m to manage patients and help with research to contain the spread of the virus.

Authorities have blamed a waste management crisis in the capital for the unusually large number of infections.

Sri Lanka's main rubbish tip collapsed in April, crushing dozens of homes and killing 32 people.