Indonesia's disaster agency warned Wednesday that neighbouring Singapore and Malaysia could be hit by more choking smog, after a huge jump in forest fires in a province at the centre of an air pollution crisis last year.

Fires in Riau province, on Indonesia's western Sumatra island, caused the worst haze in Southeast Asia for more than a decade in June last year, affecting daily life for millions and sparking a heated diplomatic row.

June is the start of the forest fire season -- when slash-and-burn techniques are used to clear land quickly and cheaply, often for palm oil plantations -- and disaster officials said the number of blazes in Riau was rising quickly.

A total of 366 "hotspots" -- either forest fires or areas likely to soon go up in flames -- had been detected in the province on Wednesday, up from 97 the previous day, according to disaster agency spokesman Sutopo Purwo Nugroho.

"We must be on alert as the wind is travelling east-northeast. The likelihood of the smog reaching Singapore and Malaysia is getting higher," Nugroho said.

Experts have said that an expected El Nino weather phenomenon later this year is likely to fan the forest fires as conditions become drier than usual.

El Nino drags precipitation across the Pacific Ocean, leaving countries including Indonesia drier and parts of the Americas wetter.

But the latest outbreak of forest fires was yet to have any serious impact on daily life in Sumatra, and the skies over Singapore were still free of haze.

Authorities said that most of the forest fires last year were deliberately lit to clear land. Slash-and-burn is a traditional farming technique, but environmental groups also accuse big companies of using the method.

According to the Washington-based World Resources Institute, a large number of the recent fires have been linked to paper and palm oil giants and their suppliers.

Using data from satellite mapping tools, it found 75 hotspots between Saturday and Monday in areas where Asia Pulp & Paper (APP), one of the world's largest paper companies, has operations.

A further 43 hotspots were found in areas occupied by suppliers to paper company Asia Pacific Resources International Limited (APRIL) in the same period.

Aida Greenbury, APP managing director of sustainability and stakeholder engagement, said 10 fires had occurred on forest land run by APP's suppliers in the past five days, all of which had been extinguished.

"We do not practise and highly condemn slash-and-burn activity," she said.

APRIL said it had agreed to support the fire-fighting effort, lending its water pumps and a company helicopter.