Indonesian forest fires that have caused choking smoke to drift across Southeast Asia are spreading to new areas and are unlikely to be put out until next year, experts said on Monday.

Indonesia has come under increased pressure from its neighbours to contain the annual "haze" crisis, which is caused by slash-and-burn agriculture practices, largely on Sumatra and Kalimantan.

But it has failed to put out the fires, with "hot spots" growing in eastern parts of the country and industry officials and analysts estimating the smoke will last until early 2016.

"Maybe it will last until December and January," said Herry Purnomo, a scientist at the Center for International Forestry Research, adding that hot spots had reached Papua, a region that usually avoids widespread fires.

"It is because people are opening new agriculture areas, like palm oil," he said.

A senior official at a company active in Indonesia's forested areas said the haze could continue until March.

Indonesia usually enters its wet season in October and November, but this year the country is expected to face moderate El Nino dry conditions which could strengthen until December and may hinder efforts to control the fires.

Indonesia's national disaster management agency has made several forecasts for when the forest fires will be brought under control, many of which have now passed, but their latest target date is early November.

On the ground, NASA satellites detected 1,729 fire alerts across Indonesia on Wednesday, a national holiday, more than any single day in the last two years.

About half of the fires during the last week have been on carbon-rich peat land areas, mostly in South Sumatra, South and Central Kalimantan, Sulawesi and Papua.

Indonesian President Joko Widodo has increased government efforts to tackle the haze in recent weeks, making several visits to the worst-hit areas and asking other countries for help, but apparently to little avail.

"We all know that the burned areas are now widening beyond normal conditions," Widodo told reporters on Sunday. "...the efforts to extinguish the fires are ongoing now both by land and air. We have to be patient because the burned areas is now wide."