Hundreds of Thai firefighters and soldiers battled a forest blaze less than 100 kilometres (60 miles) from Bangkok on Friday as the kingdom grapples with air pollution that has made nearly 2 million people ill this year.

The fire has affected at least 800,000 square metres (200 acres) of forest across three hills in Nakhon Nayok province, to the northeast of the Thai capital, with two districts declared emergency zones.

The blaze comes as Thailand grapples with a spike in pollution caused in part by agricultural burning that has seen the air in some northern cities rank among the dirtiest and most hazardous in the world this week.

Since the start of the year, more than 1.7 million people have needed hospital treatment for respiratory conditions caused by air pollution, according to the latest figures from the health ministry.

Wittaya Muntpun of the forestry department said efforts to tackle the blaze were being hampered by the difficult terrain.

"This is quite a difficult scenario since the mountain is filled with sharp rocks and cliffs," he told AFP.

"The firefighters are trying to control the spread of the fire at the base of the mountain. They couldn't go high up into the mountain since the terrain is full of cliffs."

The fire is not threatening any major population centres but it is close to the edge of the Khao Yai National Park, the kingdom's oldest national park and part of a UNESCO-listed forest complex stretching to the Cambodian border.

Stubble burning

Local community leader Pachsikarn Pouyeon said the fire allegedly started from a lightning strike in the forest, which has been parched at the end of the dry season.

Authorities had tried to trigger rain on Thursday by spraying chemicals in the air, but it had not succeeded, Pachshikarn said.

Thai Prime Minister Prayut Chan-O-Cha has dispatched his interior minister to coordinate operations on the ground, his office said in a statement.

"PM Prayut and minister of defence have closely monitored the wildfire situation in Nakon Nayok and ordered relevant organisations to help putting out the fire urgently," government spokesman Anucha Burapachaisri said.

Prayut, who faces a general election on May 14, on Thursday urged the top official from regional bloc ASEAN to help coordinate efforts to reduce cross-border pollution caused by agricultural burning.

Farmers across Southeast Asia burn off stubble in fields every year after harvest time, causing widespread air pollution.

Thailand's north is particularly affected and this week the cities of Chiang Mai and Chiang Rai topped monitoring firm IQAir's worst air quality list -- beating the likes of Delhi and Beijing.