A forest fire along the mountainous border of Son La and Yen Bai Provinces strengthened last Tuesday night as it spread towards Ta Xua reserve, said Nguyen Van Luan, head of Son La Forest Management Department.

As of 5p.m. Wednesday last week, the blaze was creeping dangerously close to the protected forest, only weeks after a week-long fire in the nearby Lao Cai Province destroyed some 1,700 hectares of forest in Hoang Lien National Park.

Luan was not optimistic as he said it has been hot, dry and windy in the area.

Only around three kilometers of mountain separated the flames from the nearest, but  the rugged terrain meant that it would take firefighters 8-10 hours to reach the core of the blaze, Luan said.

Strong winds could also push the fire into natural forests in Yen Bai's Tram Tau and Van Chan districts, experts have warned.

Around 200 volunteers from Son La's Phu Yen District and another 200 from Tram Tau District have been sent to stop the fire.

The fire has destroyed around 50 hectares of forest, according to estimates by Luan's department.

Investigators said that two workers at a hydropower plant construction site in Tram Tau might have accidently caused the fire, but details on what they were suspected of having done were not yet available.

A total of 13 forest fires have occurred around Yen Bai since mid-February, said Nguyen Quang Vinh, head of the Yen Bai Forest Management Department.

Nearby Phu Tho Province also reported several forest fires last week.

Meanwhile, Lam Dong Province in the Central Highlands said more than 600,000 hectares of forests in the province were at risk of catching fire amid a prolonged dry season.

The dry season arrived a couple of weeks earlier this year and there was little rain for a period before that. Forest management authorities in many provinces in the Mekong Delta have also raised forest fire alert levels.