Indonesia has sent hundreds of firefighters to battle blazes on Sumatra island that have enveloped Singapore and Malaysia in a choking haze, senior officials said Friday.

Dozens of fires were still burning across Sumatra on Thursday -- many lit by small land holders to clear trees from areas of peatland in order to grow oil palm or other crops.

Indonesia's forest fire chief Noor Hidayat said about 300 extra firefighters had been sent to the worst-affected area, Sumatra's Riau province, which lies opposite Singapore across the Malacca Strait.

"We have been making efforts to contain the fires. It's very difficult in the peatland areas," Forestry Minister Zulkilfi Hasan told reporters.

Singapore Foreign Minister George Yeo telephoned his Indonesian counterpart Marty Natalegawa Friday to express the city state's concerns about severe air pollution, and offer help in controlling the fires.

Natalegawa "assured minister Yeo that Indonesia would address the haze problem, adding that many Indonesians in Sumatra are also affected," the Singapore foreign ministry statement said.

"This so-called haze situation is a classic case or example of a challenge that is transboundary in nature that cannot be solved simply by one country," Natalegawa said.

"It must be recalled that we have not had a similar case for more than three or four years," Natalegawa said, referring to previous success in the attempt to curb haze.

Malaysia has also been hit by the problem, which peaked on Sunday when 351 hotspots were recorded on Sumatra, but air travel has so far remained normal in all three countries.

The haze issue has resurfaced ahead of a summit meeting of Association of Southeast Asian Nations (ASEAN) leaders in Hanoi next week.

Malaysian Health Minister Liow Tiong Lai said Malaysia was in touch with the secretariat of ASEAN, which was helping establish a response.

"We are getting the ASEAN secretariat to coordinate a regional response to tackle the haze situation, which could worsen if the forest fires are not put out," he told AFP.

The most severe outbreak of haze took place in 1997-1998, when widespread fires caused nine billion dollars in economic, social and environmental damage, according to ASEAN.

Jakarta has outlawed land-clearing by fire but weak law enforcement means the ban is largely ignored in Indonesian provinces.

"Their weakest points are enforcement of the laws and lack of transparency," Joko Arif, Southeast Asia forest team leader at environmental group Greenpeace told AFP.

He said not enough information was being given to the public on the location and size of forest fires.