Helicopters dumped water on forest fires that engulfed mountains in northern Thailand.

The aircraft were dispatched to combat a blaze - reportedly sparked by lightning during a summer storm - that broke out on Khao Chaphlu mountain and spread to surrounding forests in Nakhon Nayok on Tuesday evening.

Footage shows a KA-32 helicopter from the Department of Disaster Prevention and Mitigation loading up on water to dump on the wildfires on March 30.

The department said the helicopter has flown some 20 trips to the mountains, with another helicopter from Lopburi province joining firefighting efforts today.

The Royal Thai Army and the Ministry of Natural Resources and Environment had also sent out helicopters.

Around 154,000 litres of water have been dumped on the mountains to quell the inferno. The Prommanee and Khao Phra subdistricts have been declared disaster zones as the blaze threatened to encroach on residential areas.

As of March 31, more than 300 acres of forest has been destroyed.

Authorities fear that high winds may further intensify the blaze and spread it towards the Khao Yai National Park a few kilometres away.

Thailand's Meteorological Department warned that the fire may cause severe air pollution in Bangkok and surrounding provinces if not contained in three to four days.