Haze from the fires last week blanketed neighbouring Malaysia, which has urged Jakarta to better tackle the outbreaks, an annual dry season occurrence in the archipelago nation.

About 120 personnel were already fighting fires which have caused a thick, acrid haze to hit four separate areas of Sumatra's Riau province, said Khairul Zainal, the head of the province's environmental impact agency.

He said this number would shortly rise to 600.

"They are tasked with dousing the fires and arresting people who illegally clear land," Zainal told AFP.

The fires have burned at least 2,000 hectares (4,940 acres) of forest and plantation grounds since last month, he said.

The forestry ministry meanwhile said an additional 375 men would reinforce 1,500 firefighters who have been battling fires in South Sumatra and West Kalimantan, on the Indonesian portion of Borneo island, for several weeks.

Ministry spokesman Masyud defended Indonesia's response to the fires.

"Indonesia has done its best and with or without Malaysia's help we will continue to do our best to handle forest and ground fires," he said.

Visibility in Riau's main town of Pekanbaru early Wednesday was at just 700 metres (2,310 feet), clearing to 1,000 metres by two hours later, said Syamsuddin, an official with Riau's meterological agency.

He said data showed that about 180 hotspots were burning in Riau alone.

Burning in Indonesia and some parts of Malaysia to clear land for crops causes an annual haze that has in the past smothered parts of Malaysia, Singapore and Thailand as well as Indonesia itself.

The Indonesian government has outlawed land clearing by fire but weak enforcement means the ban is largely ignored.