Firefighters battling blazes on Indonesia's Sumatra island that have cloaked Singapore in record-breaking levels of smog are "overwhelmed" and unable to cope, an official said Friday.

"We have been fighting fires 24 hours a day for two weeks," Ahmad Saerozi, the head of the natural resources conservation agency in Riau province, where the fires are centred, told AFP.

"We are overwhelmed and in a state of emergency," he said. He was referring to efforts by more than 160 firefighters in the Bengkalis district where the biggest fires are burning, mostly underground in peatland.

"Aircraft must drop water as soon as possible. We can't do this alone," he said.

Indonesia's national disaster agency said on Friday that two helicopters had been dispatched with cloud-seeding equipment to prompt rainfall, and it was hoped they could start operating later in the day.

Singapore's pollution index hit the critical 400 level at 11:00 am Friday, an all-time record making the haze potentially life-threatening to ill and elderly people, according to a government monitoring site. Acrid smoke was seeping into residential and commercial buildings across the city-state.

Saerozi said the fires were mostly in peat that was three to four metres (10 to 13 feet) deep.

"It is still burning under the surface so we have to stick a hose into the peat to douse the fire," he said.

"We take one to two hours to clear a hectare, and by then another fire has started elsewhere. The sea breeze is also blowing the fires to other areas."

Extra personnel from the conservation agency, the local government and palm oil plantation employees had been drafted in to help tackle the blazes, he said.