Vast swathes of land are burning in western Riau province and prevailing winds may continue blowing the smoke towards Malaysia and Singapore.
Indonesia's weather authorities gave the update yesterday along with a warning that the fires would be hard to put out given the dry and hot conditions.
About 30 hot spots were detected in Riau yesterday, up from 17 on Friday and just seven on Thursday.
"Satellite imagery has shown that the distribution of smoke is quite concentrated in the western part of Riau province, indicating the presence of land fires over a wide area," the Indonesian meteorology, climatology and geophysics agency, or BMKG, said in a statement.
"With the winds blowing towards the east, the smoke will potentially continue to spread to neighbouring countries such as Malaysia and Singapore."
It is "very easy" for land and forest fires to break out in the western part of Sumatra, where Riau province is located, but bringing them under control will be "difficult to very difficult", the statement said.
BMKG Pekanbaru chief Sugarin told The Sunday Times yesterday: "Dry conditions and high temperatures mean that land and forest fires can happen easily and spread quickly, making the blazes uncontrollable."
Besides Riau, other affected provinces include North Sumatra, Jambi, Aceh, West Sumatra, Bengkulu and South Sumatra.
The BMKG has urged people to stop "burning activities" and residents affected by the smoke to wear masks and stay healthy.
Satellites belonging to the National Space and Aviation Agency of Indonesia (Lapan) detected 30 hot spots in Riau yesterday morning. Across the country, 108 hot spots were detected, said Indonesia's disaster management agency (BNPB) spokesman Sutopo Purwo Nugroho.
He had previously said a combination of dry conditions, sporadic rains and illegal land-clearing by burning in the past week was responsible for the spike.
About 7,200 firefighters have been deployed in Riau, together with seven aircraft for waterbombing and cloud-seeding, he said, adding that about 3,218ha of land in the province had been razed since January.
BNPB chief Willem Rampangilei said the government is "going all out" to put out the fires.
"September is the peak of the dry season so we will heighten prevention (efforts)," he added.