HILO, Hawai'i — Big Island fire crews used helicopters for water drops and bulldozers to carve firebreaks in a battle Tuesday against a brushfire burning about six miles south of Waimea.

Winds of 35 mph or more were fanning the flames and pushing the fire toward Waimea, and fire crews were working to protect an isolated home near the intersection of Waikoloa Road and Hawai'i Belt Road that was being threatened by the fire.

The fire prompted the closure of two major highways through the center of the Big Island, and Big Island Civil Defense officials asked motorists to use alternative routes.

Waikoloa Road was closed from Mamalahoa Highway to the Waikoloa Stables because of smoke hazards and fire crews working in the area, and at 5 p.m. Mamalahoa Highway was closed from Saddle Road to Kaiminani Drive.

Fire Chief Darryl Oliveira estimated the fire had burned 300 acres or more by 6 p.m. as fire crews staged helicopter water drops from an area near the intersection of Waikoloa Road and Mamalahoa. The fire was described as about 500 feet wide as it advanced along the highway.

The fire department's two helicopters and a single civilian helicopter were making the drops, and two bulldozers were trying to confine the fire, Oliveira said.

It was not known what caused the fire, Oliveira said.

The fire was first reported at 4:10 p.m. in brush a mile south of Waikoloa Road near the intersection of Mamalahoa Highway, and then moved north to Waikoloa Road to continue burning on the other side, Oliveira said.

Firefighters extinguished the fire on the south side of Waikoloa Road, only to see the blaze north of the road shift to the east and jump Mamalahoa Highway between the 8- and 9-mile markers, he said.

In all, the fire advanced one to two miles, but no evacuations were necessary yet, said county Civil Defense Administrator Troy Kindred.

Fire crews earlier Tuesday extinguished a brushfire that burned about a half acre near the 46-mile marker of Saddle Road, and another between Mana Road and Mamalahoa Highway near Mealani Experimental Station Road.

Reach Kevin Dayton at kdayton@honoluluadvertiser.com.