A wildfire on Khao Tabaek mountain in Nakhon Nayok province has spread to a border area of Khao Yai National Park, the park chief Chaiya Huayhongthong said.
The fire, which started on the night of April 10 on the mountain in tambon Khao Phra, Muang district, has spread to nearby forest areas.
"We have 200 rangers to fight the fire and one helicopter to carry water from nearby Huai Prue reservoir, but the fire is not yet out. We will have another helicopter this afternoon to help us on the mission," he said.
The park chief also asked nearby national parks to send rangers to join the team tackling the fire.
"It is difficult to put out the fire because the area is steep and difficult to reach. In addition, dry bamboo and leaves are fuelling the fire. We need another day to bring it under control and stop it invading more forest areas in the park," he said.
Damage estimates have not yet been made, he said.
Meanwhile, more hotspots were detected in the northern region, according to a Geo-Informatics and Space Technology Development Agency (Gistda) report.
The Suomi NPP satellite on Tuesday showed 1,956 hotspots were detected in the country. About 930 were found in forest reserves. Chiang Mai recorded the highest number at 382, followed by Chiang Rai (216) and Nan (197).
Myanmar still holds the record for the most hotspots in Southeast Asia at 14,004, followed by Laos with 6,878, according to Gistda.
The authorities say the public should be aware of hazardous levels of ultra-fine PM2.5 dust accompanying the increase in hotspots.
The average PM2.5 levels in at least 30 provinces, mostly in the northern and northeastern regions, remain higher than the safe threshold of 50 micrograms per cubic metre, according to Gistda.
Gistda said it will apply a completed THEOS-2 system, which focuses on natural disaster analysis and tracking.
Chiang Rai governor Puttipong Sirimat, as a director of Chiang Rai's Haze and Wildfire Prevention Centre, said many wildfires in the province were still being reported despite a 60-day burning ban on Feb 15.
Of the 191 hotspots detected on Wednesday, most were in Mae Suai district (59), followed by Wiang Pa Pao district (43) and Muang district (19).
Meanwhile, the province's PM2.5 density levels fell, with the haze on Tuesday evening recorded at 146 µg/m³, still above the safe threshold.
Mr Puttipong said the authorities are helping put out fires by erecting barriers and dropping water from helicopters. They agreed to extend the burning ban for 15 more days to help ease pressure on authorities, said Mr Puttipong.
Prime Minister Prayut Chan-o-cha said he wants more cooperation regarding the haze.
He showed no sign of being worried after 1,700 Chiang Mai residents filed a lawsuit with the Administrative Court on Monday against him for poor haze management, adding they should also be aware of the burning of weeds, which contributed to the haze problem.