The fire broke out at around 8 pm on Saturday on Cao San Mountain in Tam Chuc Ward before spreading down the mountainside into a protection forest bordering Ly Thuong Kiet Ward.
Local authorities, police, military personnel, forest rangers and residents brought the blaze largely under control by 10.30 pm.
However, the fire flared up again at around 10 am on Sunday, prompting authorities to evacuate households at risk and temporarily halt traffic on a section of National Highway 21A.
About 200 people, supported by fire engines and firefighting equipment, were deployed to contain the blaze.
Preliminary findings indicate the fire was sparked by a lightning strike on the mountain peak before spreading down the slope.
As of 2 pm on Sunday, about 3,000-3,500 square metres of forest, mainly undergrowth, shrubs and climbing plants, had been damaged.
Authorities were continuing firefighting efforts and monitoring the situation.
