The number of new dengue cases has fallen sharply in the face of an all-out effort to stem the spread of the mosquito-borne virus.

Last week, there were 412 new cases, 100 cases fewer than in the week before, though still significantly more than is normal at this time of the year.

Even in 2013 - when dengue cases surged to an all-time high of 22,000 - only 247 cases were reported in the same period.

On its website, the National Environment Agency (NEA) advised people to keep up the fight against dengue, warning that the number of cases this year could exceed 30,000.

The projection is based in part on the 5,000 new dengue cases detected since the beginning of the year.

Last month, NEA announced that it would be stepping up its anti-dengue efforts, including training more than 5,000 additional grassroots volunteers to educate residents on preventing mosquito breeding.

It will also roll out another 20,000 Gravitraps - placed in common corridors to trap mosquitoes - by June this year.

The agency also announced home owners will be fined $200 if they are found to be breeding mosquitoes at home. Previously, only those whose homes were in active dengue clusters were penalised.

This was an apparent response to NEA's discovery of 50 per cent more mosquito-breeding sites in homes in January, compared with numbers for the same month last year. As of Tuesday, there were 102 active dengue clusters in Singapore, where at least two cases were reported within 150m of each other in a fortnight.

Of these, 22 were considered high-risk red zones.

The largest, with 102 cases, is in the area bounded by Lorong Chuan, Ang Mo Kio Avenue 1 and the Central Expressway.

Since the start of the year, stop-work orders have been issued to construction companies at 19 sites.

This year's higher-than-usual dengue numbers are attributable to a change in the prevailing dengue virus, as well as warmer weather in recent months, which facilitates mosquito-breeding.

Dengue expert Tikki Pang, a visiting professor at the Lee Kuan Yew School of Public Policy at the National University of Singapore, said that dengue tends to be difficult to control because mosquito-breeding spots can be easily missed.

"There may be many breeding sites that we don't know about," he said. "Take construction sites, for example. They are so messy, and each one is different."