Thousands of Sri Lankan troops Friday joined a massive clean up operation to eliminate mosquito-breeding grounds as part of a national effort to contain the dengue virus, officials said.

Soldiers removed garbage piles and helped clear blocked drains in public schools to fight dengue, which is spread by mosquitoes, after more than 75 deaths from the disease this year.

"The clean up operation began in schools with the help of the military," health ministry spokesman Dharma Wanninayake said. "The programme is being extended across the country in the coming days."

Official figures show that 15,000 people were infected with dengue in the first five months of this year compared to 10,300 in the corresponding period last year.

However, health officials say the numbers could be much higher because many sufferers are not counted or seek treatment from private hospitals.

Dengue fever causes severe flu-like symptoms and there are no specific medications available to treat the disease.

Most people recover within two weeks, but it can be fatal if it leads to haemorrhaging.