SINGAPORE, Aug. 15 - Smoke from burning jungles in Indonesia spread across wider portions of Southeast Asia on Monday, as firefighters from Malaysia moved in to help combat the string of blazes that are testing relations between the neighbors.

An acrid haze covered some Indonesian cities and drifted up the northern coast of Borneo and north along the Malay Peninsula into Thailand, where officials issued health warnings.

Satellite imagery showed the greatest concentration of visible fires to be in the western part of Kalimantan Province, in the Indonesian part of Borneo, spewing the haze that has become an annual scourge for the region as poor farmers, illegal loggers and plantation companies in Indonesia set hundreds of fires to clear land.

Last week, the fires forced Malaysia to declare a state of emergency in two coastal cities as haze levels there reached record highs. Low visibility and pollution forced officials to close schools and shut down Malaysia's busiest port for a day.

Still, while the intensity of this month's haze was more severe in some spots than ever, its overall impact appeared to be less than that of the haze that covered Southeast Asia in 1997. That episode hospitalized 40,000 people and caused what the United Nations has estimated was $9.3 billion in health costs, lost agricultural production and other economic damage.

Few hospitalizations related to this year's haze have been reported so far. And aside from the one-day closure of Port Kelang in Malaysia, the heavy smoke has had little impact on traffic through the Strait of Malacca, one of the world's busiest sea lanes. Port authorities and oil traders in Singapore said no delays had been reported. Air traffic also appears to be operating normally.

But the risk that Indonesia's fires could spread remained high. The northern coast of Sumatra and the southern coast of Borneo are extremely dry, according to the Malaysian Meteorological Service. Once fires start, underground peat and coal seams can keep them smoldering for weeks.

Although natural conditions increase the risk, most of the fires were caused by human activity. Indeed, most lie far from drought-afflicted areas.

In Malaysia, authorities have begun to look beyond the immediate threat to how to prevent its annual recurrence, including taking legal action against plantation companies that worsen the haze by burning trash around the capital, Kuala Lumpur. Government officials were also reported to be investigating allegations that Malaysian plantation companies with property in Sumatra might also be involved in setting fires there.

Malaysia's team of 125 firefighters arrived Monday in the capital of Indonesia's Riau Province, Pekanbaru, a city best known as a base for oil companies like Chevron that operate in the petroleum-rich province.

Provincial authorities have long complained that too little of the oil being pumped out of Riau was benefiting its own people, doing little to alleviate the poverty that contributes to the haze problem. Poor farmers, lacking modern machinery, still rely on slash-and-burn techniques to clear new land for farming.

Authorities also lack adequate equipment to fight the fires and have reportedly resorted to bucket brigades. Malaysia's firefighters carried in portable, lightweight pumps, hoses, nozzles and other essential equipment, according to Norizan Sulaiman, a spokesman for the Malaysian Fire and Rescue Department in the government's administrative center, Putrajaya.